definition of an object (point for point) difficult of attainment. 3. The function of angular aperture of the objective. And he maintains that this latter part of the subject opens an entirely new aspect of the theory of the microscope which has been hitherto ignored.

Whether a point or line of light, or shadow, can be seen, has absolutely nothing in common with the question whether that which is seen well (and which is, of course, the optical expression of some fact) as an isolated object, can be equally well seen when in close apposition with a series of similar and equally minute objects. We cannot longer continue to interpret on the strength of an hypothesis which has been disproved.

In the grosser cases of bad definition nobody pretends that dispersion circles adequately represent the object under view, but neither could anyone charge upon this explanation (dispersion) that it proved or implied any limitation of visual power.'

6

And, in considering the subtle phenomena conditioned by the use of objectives constructed with large aperture, if it be proved that their optical function labours under various physical disabilities-among others, for instance, that rays of light may so interfere as to render void instead of presenting to the eye the actual structure from which they arise this is surely no reason for confounding the fact that a structure cannot be seen under these given conditions, with the inference that an ultimate limit of vision' has been found for all other objects and under all other circumstances. It seems more natural to believe that minute particles in close juxtaposition shall, when viewed through a microscope, produce such interference of rays as to annul or distort the microscopic image; although the same, or even much smaller particles, when isolated (not interfering with each other) may be distinguished.

The flagellum was seen, not resolved-if the term 'resolution' has any meaning!

1

Abbe does assert that what has been, or can be resolved by a 5, 6, or inch objective can be as clearly resolved by a or immersion lens, because the additional amplification helps at furthest to enlarge the image, but it is an empty amplification. Take Dr. Woodward's photographs of Diatomacea with a . Have the possessors of 35, or objectives seen anything more or newer than Mr. Stephenson's Pleurosigma drawing from 4-inch immersion?

Professor Abbe's whole writings sufficiently prove that he has kept to the subject of geometric definition of images in the microscope, diffraction images, distribution of light in the microscopic image, illumination, &c.; and these are so bound up together, and the theory of the microscope and the theory of illumination are so interconnected, that neither could be understood without the other.

But the question of the minutest dot or line which can be seen is simply one of physiology; thus, what is the least difference of light and shadow disposed in line or as isolated lights or shadows (white on dark or dark on white) which, thrown on the retina, can excite special nerve sensation, i. e. sight? I exclude colour, because differences of colour are more perceptible than grades of more or less white light.

And as a mere triumph of illumination and manipulation, the glimpsing a monad flagellum deserves attention. But what is the value (I mean of course the microscopic not the anatomical value, with which I am not dealing) of the discovery of an indistinct, shadowy line, without any interior, so to speak, to resolve '?"

The Animal of Millepora alcicornis.-The attention of zoologists was called to the relations of Millepora by the announcement of Agassiz, in 1858, that "Millepora is not an Actinoid Polyp, but a genuine Hydroid, allied to Hydractinia." Professor Agassiz figured the animals as seen by him, in his 'Contributions to the Natural History of the United States,' vol. iii. p. 61. On the evidence afforded by a single observation of Millepora, he proposed to transfer to the Acalephæ not only that genus, but all the Madreporaria Tabulata of Milne-Edwards. Professor Verrill has shown that the latter inference cannot be accepted, and that the Madreporaria Tabulata form an artificial and quite heterogeneous assemblage. There has been much difference of opinion as to the soundness of Agassiz's conclusion in regard to Millepora itself, and the extreme shyness of the animals. has rendered it impossible to accumulate numerous observations. A paper by General Nelson and P. Martin Duncan † contains figures of the animals of Millepora alcicornis as observed by the former author while stationed at Bermuda many years ago. The figures differ from those of Agassiz in arranging the tentacles regularly in whorls of four, and the authors conclude that Millepora is probably an Alcyonarian. The arrangement of tentacles is certainly quite unusual in the Alcyonaria, admitting the correctness of General Nelson's figures. In November, 1875, a paper by Mr. Moseley, of the Challenger expedition, was read before the Royal Society, in which the author reported observations on Millepora at Bermuda and elsewhere. The observations seem to have been quite unsatisfactory, and the author at that time ventured no conclusion from them. He was, however, more fortunate at Tahiti; and his paper read before the Royal Society in April, 1876, § gives the results of a more complete and satisfactory series of observations on the genus in question than has been made by any other author. His conclusions agree substantially with those of Agassiz.

In the winter of 1876-7 the writer spent some weeks in Bermuda, residing for a part of the time at Flats Village, on the shore of Harrington Sound. The abundance of Millepora in the shallow water of that beautiful lagoon afforded excellent opportunity for an investigation of the animals. In this work the writer was favoured with the kind assistance of Mr. G. Brown Goode, of the Smithsonian Institution. Our experience enabled us to appreciate the difficulty which observers have always found in the extreme shyness of the animals. Great care was taken, in collecting the animals, to avoid

* It is, of course, obvious that with lines of an inch in width, and interspaces of the same dimensions, there would be only 100,000 lines in an inch of space.

Ann. and Mag. Nat. Hist.,' xvii. 354.
Phil. Trans.,' clxvi. 91.

§ Ibid., clxvii. 117.

In

subjecting them to any more of a shock than was necessary. accordance with a suggestion of Professor Verrill, we were careful not to have the specimens out of water for an instant either in collecting them or in the subsequent manipulation. Specimens were collected at various hours of the day, and examined at about all hours of the day and night. Only once were we favoured with a sight of the zooids in expansion. Though that observation was far from being as satisfactory as could be desired, the writer has thought it might be worth while to give an account of it; for, on a subject so important and presenting such difficulties to every observer, every scrap of observation is probably worth saving.

The zooids which we saw in expansion showed generally a pretty regular whorl of tentacles at the summit. There seemed to be indications of a tendency to a grouping of the tentacles in one or more whorls below the one at the summit. But these lower whorls were not at all regularly developed, and in some cases the tentacles were scattered singly without any recognizable arrangement in successive whorls. Where an approximation to a whorled arrangement could be recognized, the number of tentacles in a whorl was generally four, but appeared to be sometimes three. As regards the arrangement of the tentacles, our observation is therefore substantially in agreement with those of Agassiz and Moseley. We feel very confident that the tentacles are not in uniform and regular whorls of four, as figured by Nelson and Duncan.

f t I & I

1.

2.

3.

4.

5.

The accompanying figures, 1 to 20, represent the outlines of several zooids in the various positions in which they chanced to present themselves. The drawings were made hastily while the specimens were under examination. It is needless to remark that they make no pre

tention to any artistic character. Whatever value they may have arises from the fact of a conscientious endeavour to draw exactly the outlines which were seen, not a line being added hypothetically or inferentially.

Figs. 1 to 16 represent zooids seen more or less nearly in profile; Figs. 17 to 20, zooids seen from above. Figs. 5, 6, 8, 14, 15, were

TITTT

11.

12.

13.

14.

15.

drawn by Mr. Goode; the remainder by the writer. The drawings testify to the entire agreement between the two observers. The zooids seen by us appear to have been of the mouthless kind. Moseley has noticed the fact that these expand much more readily than the others. Our observations were made partly with a 2-inch, but chiefly with a 1-inch objective.

Some attempts were made to study the zooids by means of decalcified specimens, previously treated with picric acid and alcohol; a preliminary treatment with picric acid and subsequent removal to alcohol having been shown by experiments undertaken by members of the United States Fish Commission in 1874, to be remarkably effective in preserving the delicate tissues of Hydrozoa. We did not succeed in obtaining by this means any zooids in satisfactory condition. The specimens, however, prepared as above stated, and subsequently mounted in glycerine jelly, show well some details of structure, particularly the lasso-cells with extraordinarily long threads, figured by Moseley.* Moseley's figures of a lasso-cell from Millepora nodosa illustrates well the character of those in Millepora alcicornis, though in the latter the spinous portion is somewhat nearer the base of the thread. The length of the thread in the largest of our specimens is about ·027 inch.†

In

Liberation of the Zoospores, Antherozoids, &c., in the Lower Plants. —M. Cornu has succeeded in producing at will the emission of the antherozoids of Polystichium Filix-Mas, of which the prothallia had been kept at the surrounding temperature during the cold season. March, one of the prothallia, having been removed and placed in a drop of the liquid from the same flask, to be examined under the microscope, emitted a great number of agile antherozoids; the same thing happened in June. Nothing was changed in the conditions of * Phil. Trans.,' clxvii. pl. ii. fig. 1.

† Mr. Wm. North Rice, in American Journal of Science and Arts,' vol. xvi.

p. 180.

existence of the antheridia-medium, light, temperature-only one new influence could have affected them, that of the air. M. Cornu reported several years ago an analogous fact* amongst the aquatic fungi. The result of his observations is, that the conditions sufficient to allow of the complete and definitive development of the antheridia and sporangia may be insufficient to allow of the dehiscence. This dehiscence is not a violent result of endosmose, since it remains suspended during long intervals, the prothallus being immersed in a liquid; it is not determined by the variations of temperature or the intensity of light, for no change of this nature was produced in the experiment. If the zoospores of the Algæ escape in the early hours of clear days in spring or summer, it is because the water which contains them becomes richer in oxygen under the action of the chlorophyll when exposed to the light.

We are thus led to conclude that the aeration of the water gives to the already formed agile corpuscles a sufficient energy to enable them to free themselves. Heat produces analogous effects. The Edogonia, which, placed in a room at 7 or 8 degrees, do not emit their spores, produce them abundantly when they are transferred to an atmosphere of 16 or 18 degrees.

M. Cornu thinks that air or heat acts by increasing the activity of the plasmatic movements, and that it is in consequence of an activity natural to the protoplasm-which is destitute of membrane, and, in spite of that, capable of utilizing oxygen-that the wall of the zoosporangium is perforated.†

Ecistes pilula.-In the Monthly Microscopical Journal,' Mr. C. Cubitt refers to this species under the name of Melicerta pilula, which he gave to it" from the fact that she fortifies the gelatinous basis of the theca with her own excremental pilules." Mr. A. W. Wills, who has very recently had opportunities of observing the rotifer, says:

"Mr. Cubitt's description of the singular habit of this animal is quite correct, but he does not appear to have observed the precise manner in which the remarkable operation is performed, from which it derives its name. It is self-evident that only a minority of the excrementary pellets discharged by the creature can be required or used to fortify its theca. The larger part are whirled away from the vicinity of the animal, in the manner familiar to all who have observed the thecated Rotifers or the fresh-water Polyzoa; but those which are utilized for building purposes are ejected between the rotifer and its tube or theca, and received under the lower margin of the ciliated trochus, where they remain for a few seconds, as if the animal were making sure of its proper hold, and then, by a sudden retraction of its body, it dabs the pellet into a proper position on the margin of the theca, and instantly resumes its usual condition. The amount and regularity of the pellets with which the tube is fortified varies very much. One finds occasionally an individual in which they See his "Monographie des Saprolegniées" (Ann. Sc. Nat.,' 1872, vol. xv. p. 117).

+ Bull. de la Soc. Bot. de France, 'vol. xv. p. 39. (From 'Comptes Rendus.') Vol. viii. p. 5.