cyst, as it appears in the drawing, did not exhibit any trace of the peristome, but the contractile bladder was seen at c. The nucleus had taken the form shown at n in one instance, and in another instance was contracted into a spherical mass. The body stripes were plain, and the sparsely scattered grain of chlorophyll pale yellow-green.

Stein describes the encysting process as taking place when the creatures are strongly contracted. They surround the whole of their bodies, except the hinder end, with thick layers of the gelatinous matter, and when this has hardened, they close the aperture. Quite different is the formation of the gelatinous dwelling-tubes, which he has occasionally found to be constructed by the colourless specimens of S. polymorphus, but frequently by S. Roeselli and other species.

The process of multiplication by fission is very common, though ordinarily it is only seen in the first stage, in which the Stentors remain a long time. It commences by the formation, on the left ventral side, of a narrow longitudinal protuberance, which begins close behind the peristome of the mother creature, near the peristome angle, and proceeds somewhat obliquely leftwards and backwards to nearly the middle of the left side of the body, Plate I., Fig. 10, p'. This is the first step towards a new peristome. At the commencement the longitudinal protuberance which represents the future oral ciliary wreath is nearly straight or slightly bowed, and furnished with very delicate cilia; but its lower end soon curves inwards, while the upper portion takes the form of a broad band, from which the cilia grow with an outward direction.

Before the cilia have obtained their full size and strength, a half-moon-shaped cleft appears in the lower curved end of the protuberance, which gradually widens and becomes the mouth, rolling itself inwards in a left-handed spiral.

Stein describes and figures the nucleus of the mother Stentors as preserving its pearl chaplet form until the mouth slit of the new creature is formed, when its several beads coalesce, and form a broad elongated oval body of homogeneous transparent material, and without trace of any granules.

A change likewise occurs in the water canal system of the mother creature. The long canal is divided, and a new contractile vesicle formed just below the new peristome. The new contractile vesicle is filled from the long canal, and sends its contents to the old vesicle, from which they escape through the anus. As the

division proceeds the new peristome becomes more distinctly divided from the maternal peristome, and the nucleus stretches out to a narrow band bent irregularly in and out, and reduced in the middle to a mere thread. Stein was not able to follow the whole of the fission process. Stein says the conjugation of the Stentors takes place with the left halves of their peristome-field.

On the 24th of May, Stein discovered in a pond two very large Stentors, filled with deep green globules of chlorophyll, and which, in addition to the ordinary chaplet-formed nucleus exhibited distinct embryo cells, but no trace of food. The nuclei in these individuals was composed of twelve homogeneous beads in one case, and eight in the other. The embryo cell, like that of the vorticellæ, consisted in a colourless transparent substance, with a small contractile vesicle near the periphery, and an opaque kernel in the centre; and in one of them this kernel had several coarse granules embedded in it. The embryo cells were larger than the nucleus segments.

On the 4th and 6th of June following the preceding discovery, Stein obtained from the same locality four large green Stentors, which had no nuclei, but bodies like the embryo cells, but without contractile vesicles, which he designates" germ-cells" (keim kügeln). One of the creatures had four of these bodies, two had five, and one six. Their diameter was 1-75′′ to 1-63′′. Each one exhibited a round central opaque nucleus 1-190′′-1—125′′ in diameter, below which a second smaller number was often seen. In the individual with five germ-cells, ten highly refractive homogeneous little balls, 1-150"" in diameter, and like flat globules, were scattered amongst them. He could not determine whether these bodies belonged to the nucleus.

:

Stentor cæruleus is described by Stein as having a body reaching a large size in fully outstretched specimens one-third as wide in front as long. The whole material of the body coloured a more or less deep blue. The nucleus a distinctly jointed rose-wreath shaped band. It reaches the same size as S. polymorphus, and there is no striking difference in its peristome, mouth, anus, contractile vesicle, or long canal. Its structure is, however, much softer, and more yielding than that of S. polymorphus. The blue colour constitutes the chief distinctive character. The colour resides chiefly in the stripes, the narrow spaces between them and the inner parenchyma being almost colourless. In some cases the colour is so pale that it disappears under the strong illumination used with the microscope, and such individuals might be taken for

colourless specimens of S. polymorphus, but the latter appear white or yellowish white by reflected light, while the pale cœruleans exhibit under the same circumstances a blue tint.

In some specimens Stein found the peristome and upper portion of a fine blue, and rest of the body colourless. In no case has he found chlorophyll globules in S. coeruleus, but he has met with much larger green bodies in those having the appearance of belonging to the monads.

In the inner parenchyma a great number of irregular, round, oval, or disc-shaped vacuoles are commonly seen, sometimes filled with water only, and sometimes with objects swallowed as food. In some instances each vacuole contained an Euglena viridis, giving a green spotted aspect to the whole creature. For a considerable time these Euglenæ remained alive, making their customary contractions and expansions before they perished and were digested. Their appearances are shown in Plate I., Fig. 9, as are also spindleshaped vacuoles, v, v, v, filled with tufts of jointed threads of algæ. The observation of these threads, Stein considers renders it probable that what Claparède and Lachmann took for spermatozoa were only masses of vibrions. Although the nucleus generally resembles that of S. polymorphus, it presents peculiarities tending to show that cœruleus is a distinct species. The nucleus beads are mostly of a long spindle shape, and separated by larger commissural threads n, n, Plate I., Fig 9. The number of the beads is most frequently from seven to nine, never more than thirteen, or less than four. Young individuals possess a single oval nucleus.

(To be continued.)

WORK FOR THE TELESCOPE.

BY THE REV. T. W. WEBB, M.A., F.R.A.S.

THE detection of extremely minute stars or satellites, especially in the immediate vicinity of a more brilliant source of light, is not perhaps a very eligible criterion of the perfection of a telescope. Such feeble points are so easily extinguished by the least want of translucency in the atmosphere, that they are deficient in the stability and uniformity which a test ought to possess; and they are as much, or possibly still more, under the influence of what is technically called "personal equation," or, in common phrase, the different powers of vision in different individuals. Every one who has been accustomed to use a telescope in company with others must have remarked instances of such disagreement, arising occasionally from want of practice in this peculiar employment of the eye, but frequently also from greater or less sensibility in the organ itself. I recollect, on one occasion, when I was examining Saturn with the splendid 8-inch Alvan Clark object-glass, formerly belonging to the late lamented Mr. Dawes, he had a distinct view of Enceladus (the innermost satellite but one), where I could not perceive a trace of it. This, however, was matter of little surprise: it was no proof of inferior vision to be beaten in any respect by that exquisitely perfect sight: but I have frequently been led to infer a greater deficiency in my own power of picking out very minute points of light, than of catching feeble nebulæ, or delicate planetary or lunar markings; and such variations, I believe, are very frequent, and may be the cause of many discrepancies in the records of observations.

For these reasons, too much dependance should not be placed on objects of this class, as satisfactory tests of telescopic perfection. They are unquestionably accurate in their indication; but it is that of a compound result in which the quality of both the atmosphere and the eye are combined with the capability of the telescope. It is only when the same eye is employed at the same time in comparing different instruments, and when two of these variable quantities are thus eliminated, that such a criterion becomes of absolute value. Still there is an interest attached in many minds to these minute explorations; and it is on this account that I mention that I have recently had an excellent view of the little companion, called by Smyth, with his 5-inch achromatic, “a glimpse star," n f the celebrated pair n Corona. I had seen it on

one occasion with a very fine 104-inch mirror by Mr. With, but had never caught it with my own of 91-inch, by the same maker. The night of April 29, 1869, being clear, and definition very fair, though not satisfactory as to steadiness, I found that this minute point, though imperceptible with 65, was glimpsed with 110, steadily seen with 212, and obvious with 450 (powers approximate). It would probably have been still easier in perfectly tranquil air.

The most difficult object of this class which I remember to have caught is the companion of μ Andromeda, seen once only by Smyth, when the large star was hidden by a bar in the field. I have often failed in perceiving it, even under apparently favourable circumstances, but on four several occasions have been able to hold it pretty steadily with 450 in the full presence of its overpowering neighbour. This star was rated 16 mag. by Smyth. Dawes considered it 11, or at the most 11 of his scale, which was that of Struve; 11 being equal, he says, theoretically to Sir J. Herschel's 17 mag., 20 being the conventional limit of his 18-inch front-view mirror. I found it in the first instance, Oct. 19, 1867, without knowing in which direction to look for it; so that it is a fair proof of the efficiency of silver on glass, both in point of light and definition. It will give an idea of the extraordinary sensitiveness of Mr. Dawes's eye to mention, that he informed me that he just got an occasional glimpse of it, without previous knowledge of its position, with an aperture of only 4 inches on his 8-inch objectglass. Yet this was at a date (Nov. 2, 1867), not long previous to his decease, when he had "become fully convinced " that his vision. was "not quite what it used to be!"

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As a comparative test of definition, independently of light, may be useful to some amateurs, I may mention that on April 29, I saw n Corona doubtfully elongated with 110 (it would probably have been decided on a better night), distinctly split with 212, and separated by a black space equal to about either disc with 450; the air, however, was not steady enough for an accurate judgment. On the following night, definition being rather less favourable, Boötis appeared round with 110, but clearly divided with 212, and wide apart, but less than a disc, with 450. The much larger size of the discs in this case (3 and 4 of Smyth against 6 and 6) accounts for the greater difficulty of this pair, even if the central distance should be the same (I have seen no recent measure of either).

But a few years ago, observations on nebula with a 9-inch silvered reflector, however individually interesting, would have been of no great value for comparison, as but few amateurs would have