20

length of the object-glass to the length of the unit of the scale. But the eye is tolerant of small changes in the focal adjustment which sensibly affect the scale-value. These changes may and do arise from the following causes: (i.) The focal length of the object-glass and the length of the tube are affected by temperature. (ii.) The focal length is sensibly different for objects of different colour. (iii.) The length of the scale is affected by temperature. (iv.) The state of adaptation of the observer's eye is dependent on his state of health, on a condition of greater or less fatigue, or on the inclination of the head threads in consequence of the altitude of the object observed. (v.) The temperature of the object-glass, of the scale and of the tube, cannot be assumed to be identical. Thus, for refined purposes, it cannot be as

Circle of position Circle of declination

9534.F

61

From Engineering, vol. xlix.

Mic for deco

Micrometer threads
For position

FIG. 16.

22

62

Cross

sumed with any certainty that the instantaneous scale-value of the heliometer is known, or that it is a function of the temperature. Of course, for many purposes, mean conditions may be adopted and mean scale-values be found which are applicable with considerable precision to small angles or to comparatively crude observations of large distances; but the highest refinement is lost unless means are provided for determining the scale-value for each observer at each epoch of observation.

In determinations of stellar or solar parallax, comparison stars, symmetrically situated with respect to the object whose parallax is sought, should be employed, in which case the instantaneous scale-value may be regarded as an unknown quantity which can be derived in the process of the computation of the results. Examples of this mode of procedure will be found, in the case of stellar parallax in the Mem. R.A.S. vol. xlviii. pp. 1-194, and in the Annals of the Cape Observatory, vol. viii. parts I and 2; and in the case of planetary parallax in the Mem. R.A.S. vol. xlvi. pp. 1-171, and in the Annals of the Cape Observatory, vol. vi. In other operations, such as the triangulation of large groups of stars, it is necessary to select a pair of standard stars, if possible near the middle of the group, and to determine the scale-value by measures of this standard distance at frequent intervals during the night (see Annals of the Cape Observatory, vol. vi. pp. 3-224). In other cases, such as the measurement of the mutual distances and position angles of the satellites of Jupiter, for derivation of the elements of the orbits of the satellites and the mass of Jupiter, reference must also be made to measures of standard stars whose relative distance and position angle is accurately determined by independent methods (see Annals of the Cape Observatory, vol. xii. part 2).

Gill introduced a powerful auxiliary to the accuracy of heliometer measures in the shape of a reversing prism placed in front of the eye-piece, between the latter and the observer's eye. If measures are made by placing the image of a star in the centre of the disk of a planet, the observer may have a tendency to do so systematically in error from some acquired habit or from natural astigmatism of the eye. But by rotating the prism 90° the image is presented entirely reversed to the eye, so that in the mean of measures made in two such positions personal error is eliminated. Similarly the prism may be used for the study and elimination of personal errors depending on the angle made by a double star with the vertical. The best plan of mounting such a prism has been found to be the FIG. 17. following. 1, 2 (fig. 17) are the eye lens and field lens respectively of a Merz positive eye-piece. In this construction the lenses are much closer together and the diaphragm for the eye is much farther from the lenses than in Ramsden's eye-piece. The prism p is fitted accurately into brass slides (care has to be taken in the construction to place the prism so that an object in the centre of the field will so remain when the eye-piece is rotated in its adapter). | There is a collar, clamped by the screw at S, which is so adjusted that the eye-piece is in focus when pushed home, in its adapter, to this collar. The prism and eye-piece are then rotated together in the adapter.

The Double Image Micrometer.-Thomas Clausen in 1841 (Ast. Nach. No. 414) proposed a form of micrometer consisting of a divided plate of parallel glass placed within the cone of rays from the object-glass at right angles to the telescope axis. One-half of

this plane remains fixed, the other half is movable. When the inclination of the movable half with respect to the axis of the telescope is changed by rotation about an axis at right angles to the plane of division, two images are produced. The amount of separation is very small, and depends on the thickness of the glass, the index of refraction and the focal length of the telescope. Angelo Secchi (Comptes rendus, xli., 1855, p. 906) gives an account of some experiments with a similar micrometer; and Ignarjio Porro (Comptes rendus, xli. p. 1058) claims the original invention and construction of such a micrometer in 1842. Clausen, however, has undoubted priority. Helmholtz in his " Ophthalmometer has employed Clausen's principle, but arranges the plates so that both move symmetrically in opposite directions with respect to the telescope axis. Should Clausen's micrometer be employed as an astronomical instrument, it would be well to adopt the improvement of Helmholtz. Double-Image Micrometers with Divided Lenses. Various micrometers have been invented besides the heliometer for measuring by double image. Ramsden's dioptric micrometer consists of a divided lens placed in the conjugate focus of the innermost lens of the erecting eye-tube of a terrestrial telescope. The inventor claimed that it would supersede the heliometer, but it has never done anything for astronomy. Dollond claims the independent invention and first construction of a similar instrument (Pearson's Practical Astronomy, ii. 182). Of these and kindred instruments only two types have proved of practical value. G. B. Amici of Modena (Mem. Soc. Ital. xvii., 1815, pp. 344-359) describes a micrometer in which a negative lens is introduced between the eye-piece and the objectglass. This lens is divided and mounted like a heliometer objectglass; the separation of the lenses produces the required double image, and is measured by a screw. W. R. Dawes very successfully used this micrometer in conjunction with a filar micrometer, and found that the precision of the measures was in this way greatly increased (Monthly Notices, vol. xviii. p. 58, and Mem. R.A.S. vol. xxxv. p. 147).

In the improved form1 of Airy's divided eye-glass micrometer (Mem. R.A.Š. vol. xv. pp. 199-209) the rays from the object-glass pass successively through lenses as follows:

The lens b is divided, and one of the segments is moved by a micrometer screw. The magnifying power is varied by changing the lens a for another in which p has a different value. The magnifying power of the eye-piece is that of a single lens of focus = p.

could be equally satisfied if the divided lens were made concave In 1850 J. B. Valz pointed out that the other optical conditions instead of convex, with the advantage of giving a larger field of view (Monthly Notices, vol. x. p. 160).

The last improvement on this instrument is mentioned in the Report of the R.A.S. council, February 1865. It consists in the introduction by Simms of a fifth lens, but no satisfactory description has ever appeared. There is only one practical published investigation of Airy's micrometer that is worthy of mention, viz. that of F. Kaiser (Annalen der Sternwarte in Leiden, iii. III-274). The reader is referred to that paper for an exhaustive history and discussion of the intrument. It is somewhat surprising that, after Kaiser's investigations, observers should continue, as many have done, to discuss their observations with this instrument as if the screw-value were constant for all angles.

1 For description of the earliest form see Cambridge Phil. Trans. vol. ii., and Greenwich Observations (1840).

2 Dawes (Monthly Notices, January 1858, and Mem. R.A.S. vol. xxxv. p. 150) suggested and used a valuable improvement for producing round images, instead of the elongated images which are otherwise inevitable when the rays pass through a divided lens of which the optical centres are not in coincidence, viz. "the introduction of a diaphragm having two circular apertures touching each other in a point coinciding with the line of collimation of the telescope, and the diameter of each aperture exactly equal to the semidiameter of the cone of rays at the distance of the diaphragm from the focal point of the object-glass." Practically the difficulty of making these diaphragms for the different powers of the exact required equality is insuperable; but, if the observer is content to lose a certain amount of light, we see no reason why they may not readily be made slightly less. Dawes found the best method for the purpose in question was to limit the aperture of the object-glass by a diaphragm having a double circular aperture, placing the line joining the centres of the circles approximately in the position angle under measurement. Dawes successfully employed the double circular aperture also with Amici's micrometer. The present writer has successfully used a similar plan in measuring position angles of a Centauri with the heliometer, viz. by placing circular diaphragms on the two segments of the object-glass.

a

Steinheil (Journal savant de Munich, Feb. 28, 1843) describes "heliomètre-oculaire which he made for the great Pulkowa refractor, the result of consultations between himself and the elder Struve. It is essentially the same in principle as Amici's micrometer, except that the divided lens is an achromatic positive instead of a negative lens. Struve (Description de l'Observatoire Central de Pulkowa, pp. 196, 197) adds a few remarks to Steinheil's description, in which he states that the images have not all desirable precisiona fault perhaps inevitable in all micrometers with divided lenses, and which is probably in this case aggravated by the fact that the rays falling upon the divided lens have considerable convergence. He, however, successfully employed the instrument in measuring double stars, so close as 1" or 2", and using a power of 300 diameters, with results that agreed satisfactorily amongst themselves and with those obtained with the filar micrometer. If Struve had employed a properly proportioned double circular diaphragm, fixed symmetrically with the axis of the telescope in front of the divided lens and turning with the micrometer, it is probable that his report on the instrument would have been still more favourable. This particular instrument has historical interest, having led Struve to some of those criticisms of the Pulkowa heliometer which ultimately bore such valuable fruit (see ante).

Ramsden (Phil. Trans. vol. xix. p. 419) suggested the division of the small speculum of a Cassegrain telescope and the production of double image by micrometric rotation of the semispecula in the plane passing through their axis. Brewster (Ency. Brit. 8th ed. vol. xiv. p. 749) proposed a plan on a like principle, by dividing the plane mirror of a Newtonian telescope. Again, in an ocular heliometer by Steinheil double image is similarly produced by a divided prism of total reflection placed in parallel rays. But practically these last three methods are failures. In the last the field is full of false light, and it is not possible to give sufficiently minute and steady separation to the images; and there are of necessity a collimator, two prisms of total reflection, and a small telescope through which the rays must pass; consequently there is great loss of light.

Micrometers Depending on Double Refraction.-To the Abbé Rochon (Jour. de phys. liii., 1801, pp. 169-198) is due the happy idea of applying the two images formed by double refraction to the construction of a micrometer. He fell upon a most ingenious plan of doubling the amount of double refraction of a prism by using two prisms of rock-crystal, so cut out of the solid as to give each the same quantity of double refraction, and yet to double the quantity in the effect produced. The combination so formed is known as Rochon's prism. Such a prism he placed between the object-glass and eye-piece of a telescope. The separation of the images increases as the prism is approached to the object-glass, and diminishes as it is approached towards the eye-piece. D. F. J. Arago (Comptes rendus, xxiv., 1847, pp. 400-402) found that in Rochon's micrometer, when the prism was approached close to the eye-piece for the measurement of very small angles, the smallest imperfections in the crystal or its surfaces were inconveniently magnified. He therefore selected for any particular measurement such a Rochon prism as when fixed between the eye and the eyepiece (i.e. where a sunshade is usually placed) would, combined with the normal eye-piece employed, bring the images about to be measured nearly in contact. He then altered the magnifying power by sliding the field lens of the eye-piece (which was fitted with a slipping tube for the purpose) along the eye-tube, till the images were brought into contact. By a scale attached to the sliding tube the magnifying power of the eye-piece was deduced, and this combined with the angle of the prism employed gave the angle measured.

If p is the refracting angle of the prism, and n the magnifying power of the eye-piece, then p/n will be the distance observed. Arago made many measures of the diameters of the planets with such a micrometer.

Dollond (Phil. Trans., 1821, pp. 101-103) describes a doubleimage micrometer of his own invention, in which a sphere of rockcrystal is substituted for the eye-lens of an ordinary eye-piece. In this instrument (figs. 18, 19) a is the sphere, placed in half-holes on the axis bb, so that when its principal axis is parallel to the axis of the telescope it gives only one image of the object. In a direction perpendicular to that axis it must be so placed that when it is moved by rotation of the axis bb the separation of the images shall be parallel to that motion. The angle of rotation is measured on

the graduated circle C. The angle between the objects measured is sin 20, where r is a constant to be determined for each magnifying power employed,1 and the angle through which the sphere has been turned from zero (i.e. from coincidence of its principal axis with that of the telescope). The maximum separation is consequently at 45° from zero. The measures can be made on both sides of zero for eliminating index error. There are considerable difficulties of construction, but these have been successfully overcome by Dollond; and in the hands of Dawes (Mem. R.A.S. xxxv. p. 144 seq.) such instruments have done valuable service. They are liable to the objection that their employment is limited to the measurement of very small angles, viz. 13" or 14" when the magnifying power is 100, and varying inversely as the power. Yet the beautiful images which these micrometers give permit the measurement of very difficult objects as a check on measures with the parallel-wire micrometer.

On the theory of the heliometer and its use consult Bessel, Astronomische Untersuchungen, vol. i.; Hansen, Ausführliche Methode mit dem Fraunhoferschen Heliometer anzustellen (Gotha, 1827); Chauvenet, Spherical and Practical Astronomy, vol. ii. (Philadelphia and London, 1876); Seeliger, Theorie des Heliometers (Leipzig, 1877); Lindsay and Gill, Dunecht Publications, vol. ii. (Dunecht, for private circulation, 1877); Gill, Mem. R.A.S. vol. xlvi. pp. 1-172, and references mentioned in the text. (D. G.)

HELIOPOLIS, one of the most ancient cities of Egypt, met with in the Bible under its native name On. It stood 5 m. E. of the Nile at the apex of the Delta. It was the principal seat of sun-worship, and in historic times its importance was entirely religious. There appear to have been two forms of the sun-god Re-Harmakhis, falcon-headed, and Etōm, human-headed; at Heliopolis in the New Kingdom-namely, Ra-Harakht, or the former was the sun in his mid-day strength, the latter the evening sun. A sacred bull was worshipped here under the name Mnevis (Eg. Mreu), and was especially connected with Etōm. The sun-god Re' (see EGYPT: Religion) was especially the royal god, the ancestor of all the Pharaohs, who therefore held the temple of Heliopolis in great honour. Each dynasty might give the first place to the god of its residence-Ptah of Memphis, Ammon of Thebes, Neith of Sais, Bubastis of Bubastis, but all alike honoured Re'. His temple became in a special degree a depository for royal records, and Herodotus states that the priests of Heliopolis were the best informed in matters of history The schools of philosophy and astronomy of all the Egyptians. are said to have been frequented by Plato and other Greek philosophers; Strabo, however, found them deserted, and the town itself almost uninhabited, although priests were still there, and cicerones for the curious traveller. The Ptolemies probably took little interest in their " father" Re', and Alexandria had eclipsed the learning of Heliopolis; thus with the withdrawal of royal favour Heliopolis quickly dwindled, and the students of native lore deserted it for other temples supported by a wealthy population of pious citizens. In Roman times obelisks were taken from its temples to adorn the northern cities of the Delta, and even across the Mediterranean to Rome. Finally the growth of Fostat and Cairo, only 6 m. to the S.W., caused the ruins to be ransacked for building materials. The site was known to the Arabs as 'Ayin esh shems, "the fountain of the sun," more recently as Tel Hisn. It has now been brought for the most part under cultivation, but the ancient city walls of crude brick are to be seen in the fields on all sides, and the position of the great temple is marked by an obelisk still standing (the earliest known, being one of a pair set up by Senwosri I., the second king of the Twelfth Dynasty) and a few granite blocks bearing the name of Rameses II.

See Strabo xvii. cap. 1. 27-28; Baedeker's Egypt. (F. LL. G.) HELIOSTAT (from Gr. Acos, the sun, orarós, fixed, set up), an instrument which will reflect the rays of the sun in a fixed direction notwithstanding the motion of the sun. The optical apparatus generally consists of a mirror mounted on an axis parallel to the axis of the earth, and rotated with the same angular velocity as the sun. This construction assumes that the sun describes daily a small circle about the pole of the celestial sphere, and ignores any diurnal variation in the declination. This variation is, however, so small that it can be neglected for most purposes.

1 Dollond provides for changing the power by sliding the lens d nearer to or farther from a.

Many forms of heliostats have been devised, the earliest having been described by Wilhelm Jacob s'Gravesande in the 3rd edition of his Physices elementa (1742). One of the simplest consists of a plane mirror rigidly connected with a revolving axis so that the angle between the normal to the mirror and the axis of the instrument equals half the sun's polar distance, the mirror being adjusted so that the normal has the same right ascension as the sun. It is easily seen that if the mirror be rotated at the same angular velocity as the sun the right ascensions will remain equal throughout the day, and therefore this device reflects the rays in the direction of the earth's axis; a second fixed mirror reflects them in any other fixed direction. Foucault's heliostat reflects the rays horizontally in any required direction. The principle of the apparatus may be explained The axis of rotation AB bears a rigidly

H

FIG. I.

by reference to fig. 1. attached rod DBC inclined to it at an angle equal to the sun's polar distance. By adjusting the right ascension of the plane ABC and rotating the axis with the angular velocity of the sun, it follows that BC will be the direction of the solar rays throughout the day. X is the mirror X is the mirror rotating about the point E, and placed so that (if EB is the horizontal direction in which the rays are to be reflected) (1) the normal CE to the mirror is jointed to BC at C and is equal in length to BE, (2) the rod DBC passes through a slot in a rod ED fixed to, and in the plane of, the mirror. Since CE equals BE these directions are equally inclined to, and coplanar with, the normal to the mirror. Hence light incident along the direction BC will be reflected along CE. Silbermann's heliostat reflects the rays in any direction. The principle may be explained by means of fig. 2. AB is the axis of rotation, BC an adjustable FIG. 2. rod as in Foucault's construction, and BD is another rod which can be set to the direction in which the rays are to be reflected. The rods BC and DB carry two small rods EF, GF jointed at F; at this joint there is a pin which slides in a slot on the rod BH, which is normal to the mirror X. The

to the framework pafe, the members of which are parallel to the incident and reflected rays SO, OR, and the diagonal pf is perpendicular to the mirror. The framework is attached to two independent circular arcs Cs and rr' having their centres at O and provided with clamps D and A on the axis F of the instrument. The arc Cs is graduated, and is set so that the angle COD equals the complement of the sun's declination. This can be effected (after setting the axis) by rotating Cs until a needle indicates true time on the hour dial B. The arc rr is set so as to reflect the rays in the required direction. The axis F of the instrument is set at an angle equal to the latitude of the place of observation and in the meridian by means of the screw K, and rotated by clockwork contained in the barrel H. The setting in the meridian is effected by turning the instrument after setting for latitude until a pin-hole aperture s and a small screen P, placed so that Ps is parallel to CO, are in a line with the sun.

Many other forms of heliostats have been designed, the chief difference consisting in the mechanical devices for maintaining the constant direction of the reflecting ray. One of the most important applications of the heliostat is as an adjunct to the newer forms of horizontal telescopes (q.v.) and in conjunction with spectroscopic telescopes in observations of eclipses.

i.e. a plant which follows the sun with its flowers or leaves, or, HELIOTROPE, or TURNSOLE, Heliotropium (Gr. OrρóжLov, according to Theophrastus (Hist. plant. vii. 15), which flowers at the summer solstice), a genus of usually more or less hairy herbs or undershrubs of the tribe Heliotropicae of the natural order Boraginaceae, having alternate, rarely almost opposite leaves; small white, lilac or blue flowers, in terminal or lateral one-sided simple or once or twice forked spikes, with a calyx of five deeply divided segments, a salvershaped, hypogynous, 5-lobed corolla, and entire 4-celled ovary; fruit 2to 4-sulcate or lobed, at length separable into four 1-seeded nutlets or into two hard 2-celled carpels. The genus contains 220 species indigenous in the temperate and warmer parts of both hemispheres. A few species are natives of Europe, as H. europaeum, which is also a naturalized species in the southern parts of North America.

The common heliotrope of English hothouses, H. peruvianum, popularly known as cherry-pie," is on account of the delicious odour of its flowers a great favourite with florists. It was introduced into Europe by the younger Jussieu, who sent seed of it from Peru

Heliotropium suaveolens. to the royal garden at Paris. About the year 1757 it was grown in England by Philip Miller from seed obtained from St Germains. H. corymbosum (also a native of Peru), which was grown in Hammersmith nurseries as early as 1812, has larger but less fragant flowers than H. peruvianum. The species commonly grown in Russian gardens is H. suaveolens, which has white, highly fragrant flowers.

Heliotropes may be propagated either from seed, or, as commonly, by means of cuttings of young growths taken an inch or two in length. Cuttings when sufficiently ripened, are struck in spring or during the summer months; when rooted they should be potted singly into small pots, using as a compost fibry loam, sandy peat and well-decomposed stable manure from an old hotbed. The plants soon require to be shifted into a pot a size larger. To secure early-flowering plants, cuttings should be struck in August, potted off before winter sets in, and kept in a warm greenhouse. In the spring larger pots should be given, and the plants shortened back to make them bushy. They require frequent shiftings during the summer, to induce them to bloom freely.

The heliotrope makes an elegant standard. The plants must in this case be allowed to send up a central shoot, and all the side growths must be pinched off until the necessary height is reached, when the shoot must be stopped and lateral growths will be produced to form the head. During winter they should

be kept somewhat dry, and in spring the ball of soil should be reduced and the plants repotted, the shoots being slightly pruned, so as to maintain a symmetrical head. When they are planted out against the walls and pillars of the greenhouse or conservatory an abundance of highly perfumed blossoms will be supplied all the year round. From the end of May till October heliotropes are excellent for massing in beds in the open air by themselves or with other plants. Many florists' varieties of the common heliotrope are known in cultivation.

Pliny (Nat. hist. xxii. 29) distinguishes two kinds of "heliotropium," the tricoccum, and a somewhat taller plant, the helioscopium; the former, it has been supposed, is Croton tinctorium, and the latter the λотρожιоν μLкрóv of Dioscorides or Heliotropium europaeum. The helioscopium, according to Pliny, was variously employed in medicine; thus the juice of the leaves with salt served for the removal of warts, whence the term herba verrucaria applied to the plant. What, from the perfume of its flowers, is sometimes called winter heliotrope, is the fragrant butterbur, or sweet-scented coltsfoot, Petasites (Tussilago) fragrans, a perennial Composite plant.

HELIOTROPE, in mineralogy, is the mineral commonly called “ bloodstone” (q.v.), and sometimes termed girasol-a name applied also to fire-opal. The name, like those of many ancient names of minerals, seems to have had a fanciful origin. According to Pliny the stone was so called because when thrown into the water it turned the sun's light falling upon it into a reflection like that of blood.

HELIOZOA, in zoology, a group of the Sarcodina (q.v.) so named by E. Haeckel, 1866. They are characterized by the radiate pseudopods, finely tapering at the apex, springing abruptly from the superficial protoplasm, containing a denser, rather permanent axial rod (figs. I (1), 2 (2); protoplasm without a clear ectoplasm or pellicle, often frothy with large vacuoles, like the alveoli of Radiolaria; nucleus 1 or numerous; skeleton absent, gelatinous or of separate siliceous fibres, plates or spicules, rarely complete and latticed; reproduction by simple fission or by brood-formation, often syngamous; form usually nearly spherical, rarely changing slowly. This group was formerly included with the Rhizopoda; but was separated from it by Haeckel on account of the character of its pseudopods, and its general adaptation to a semipelagic existence correlated with the frothy cytoplasm (fig. 1 (1)). Actinophrys sol and Actinosphaerium eichhornii (fig. 2), known as sun animalcules to the older microscopists, float freely in stagnant or slowflowing waters, and Myriophrys is able by an investment of long flagelliform cilia to swim freely. The majority, however, lurk among confervae or the light débris of the bottom ooze; and come under the head of "sapropelic " rather than pelagic organisms. The body is usually of constant spherical form in relation to the floating habit. Nuclearia, however, shows amoeboid changes of general outline. The pseudopods are retractile, the axial filament being absorbed as the filament grows shorter and thicker and disappearing when the pseudopod merges into the ectoplasm, to be reformed at the same time with the pseudopod. There is often a distinction, clear, but never sharp, between the richly vacuolate, almost frothy ectoplasm and the denser endoplasm. One or more contractile vacuoles may protrude from the ectoplasm. The endoplasm contains the nucleus or nuclei. The nucleus when single may be central or excentric: in the latter case, the endoplasm contains a clear central sphere ("centrosome") on which abut the axial filaments of the pseudopods. The ectoplasm contains, in some species, constantly (Raphidiophrys viridis) or occasionally (Actinosphaerium), green cells belonging to the genera Zoochlorella and Sphaerocystis, both probably the latter certainly-vegetative stages of a Chlamydomonad (FLAGELLATA, q.v.) and of symbiotic significance.

The Heliozoa can move by rolling over on their extended pseudopods; Acanthocystis ludibunda traversing a path of as much as twenty times its diameter in a minute, according to Penard. Several species (e.g. Raphidiophrys elegans) remain associated by the union of their pseudopods, whether into social aggregates (due to approximation) or "colonies" due to lack of separation

FIG. 1.-Heliozoa. 1. Actinophrys sol, Ehrb.; X 800. a, Foodparticle lying in a large food-vacuole; b, deep-lying finely granular protoplasm; c, axial filament of a pseudopodium extended inwards to the nucleus; d, the central nucleus; e, contractile vacuole; f, superficial much vacuolated protoplasm. 2. Clathrulina elegans, Cienk.; X 200. 3. Heterophrys marina, H. and L. X 660. a, nucleus; b, clearer protoplasm surrounding the nucleus; c, the peculiar felted envelope. 4. Raphidiophrys pallida, F. E. Schultze; X 430. a, food-particle; b, contractile vacuole; c, the nucleus; d, central granule in which all the axis-filaments of the pseudopodia meet. The tangentially disposed spicules are seen arranged in masses on the central nucleus; b, clear protoplasm around the nucleus; c, the surface. 5. Acanthocystis turfacea, Carter; X 240. a, probably more superficial protoplasm with vacuoles and chlorophyll corpuscles; d, coarser siliceous spicules; e, finer forked siliceous spicules; f, finely granular layer of protoplasm. The long pseudopodia reaching beyond the spicules are not lettered. 6. Bi-flagellate flagellula" of Acanthocystis aculeata. a, nucleus. 7. Id. of Clathrulina elegans. a, nucleus; b, granules. 8. Astrodisculus ruber, Greeff; X 320. a, red-coloured central sphere (? nucleus); b, peripheral homogeneous envelope.

after fission, is not accurately known. The multinuclear species Actinosphaerium eichhornii (fig. 2), normally apocytial (i.e. the nuclei divide repeatedly without division of the cytoplasm),