rectangular aspect, and which views I derived solely from Lord Sydney G. Osborne's slides, and I must still attribute that aspect to the pressure of the free edge of what I have called the central blade of the ramus acting on the under side of the teeth, that free edge being forced up by the pressure on the alulæ. A fact to be borne in mind is, that in the dead Conochilus and Melicerta ringens pressure usually severs the alulæ from the propelling arms of the spur at the point c, where I have severed them in the drawings, and the consequence is that in all the mounted specimens there is little to indicate at these angles any solid attachment to the alulæ or the existence of a propelling force in connection with them. In some specimens of Melicerta, however, from Redditch, Lord S. G. Osborne has sent me mountings in which I think this connection is sustained; these Melicerta from Redditch are also remarkable for Mr. Slack's conical pellets; some are actually true cones, and others are so long that they are more like ninepins than rifle bullets; they are also remarkable for javelin-headed teeth.

*

To make out the points here detailed in Conochilus I found it necessary to study it alive and dead. It is a most convenient animal to manipulate, it cannot get away, and it gives you ten or more specimens in each group, in various attitudes. You must gently clean it from debris with two fine needles. Dead specimens should be treated as suggested by Mr. Gosse in his paper (1. c.) with potash and water; with some specimens half and half is too strong, with some it is not strong enough.

As mentioned by Mr. Davis,† the red spots (two in number) are in Conochilus most remarkable organs; each has a highly refractive hemisphere resting on the flat red disk, and apparently partially sunk into it; as Mr. Davis has not drawn it, I have done so (see Fig. 12). The young female Conochilus is born alive; the jaws work in its mother's womb. It comes into the world with a lump of transparent jelly attached to its foot, which goes to augment the globular store of material which is the pedestal or home of the colony.

Everyone must agree with Mr. Davis in thinking that the disk of Conochilus is a very strange apparent departure from the ordinary run of rotifer disks. I give a diagrammatic form, Fig. 13, of the disk, which is corrected from that given by me in the eighteenth vol. of the 'M. M. J.,' p. 214, in which I made an

The method I adopted with both Melicerta and Conochilus was to lay them under a piece of microscopic glass on a plain slide, with a very little cotton wool to ease the pressure. Under a low power I then pressed them until the mastax was separated or in the right position; then by adding the liquor potassa stronger and stronger (and capillary attraction with blotting paper draws it under the glass), the body of the animal is dissolved and the mastax left free for examination under high power.

†M. M. J.,' vol. xvi. p. 1.

error, for I treated the disk as continuous all round the sinus; and if, as Mr. Davis suggests in his paper, development from some other form is to be accepted as the mode in which Conochilus attained its present appearance, then I should offer the following suggestion, namely, that in endeavouring to arrange its disk to the best advantage for picking up trifles in a crowd of fellow-creatures grouped in a spherical form, it found it necessary to arrange the collecting cilia round the sinus; and to do this, it tilted the disk and brought the lower points of the ciliary lobes like a collar forward and round its neck-just as if a sailor were to cut a large slit in his tarpaulin hat behind, at the lower edge, and then draw the two cut corners round his neck, so as nearly to meet under his chin, and keeping the hat on the back of his head, proceed to turn his face upwards (see Fig. 13). The incision under the calcars suggests where these two points of the disk may have come from. At the same time, and though I know that I am in a small minority, yet I confess I cannot see how evolution helps us in the least with such an animal; because the moment you suggest a previous form, you ask, Where is it? and I should not like to point to any rotifer sufficiently near Conochilus to justify one in looking upon it as a predecessor in title. To me, the whole rotifer world, in fact, is a wild collection of puzzling forms. There is plenty of likeness and mixture, but there are no links that I can see. There are many forms that make you think of another, but which, when closely examined, range away widely from it in some important particular. To me, the rotifers have, in fact, ever been, and are, a stumbling-block in the way of accepting the development theory as the complete and vast agency that so many now consider it, and the manducatory organs seem to me to deserve close consideration in connection with the subject. If anywhere, I should have expected to find in the rotifer world confirmation strong of the theory-gradations and links in perfect orderclassification easy and systematic; instead of which there would seem to be some other principle at work there which runs quite athwart any notion of regularity, which baffles every scheme of classification, and exhibits most unexpected complications— striking gaps where we are led to expect continuity. I cannot help anticipating that the time may come when these very same gaps, existing as they do throughout the animal kingdom, will be recognized by scientific observers exactly as the irregularities of Uranus were fixed upon by Adams and Leverrier, and accepted as indicating another and an outer force. As long as this view is overlooked, so long will science be retarded; for what we forget we fail to seek for; and we are apt to stop gaps with great names. We want a principle in direct relation to these regularly recurring hiati-a principle which, accepting the fact that the disappearance

or degradation of some forms is, as a rule, followed sooner or later by the appearance of new and higher ones, directly accounts for it, and it may be, by directly connecting together the degradation of the old and the appearance of the new. We want a principle that would lift us out of the utter confusion of mind which we are in, when we find that a vast concourse of forms, which we are told reached their present conditions by a process of alteration so gradual as to be almost or absolutely imperceptible-by a continuity of progress requiring a gigantic amount of time for its execution-is yet more especially remarkable for a succession of violent, irreconcilable gaps, occurring too at the very places where we want and are led to expect links. We need a principle in direct relation to the extraordinary gradations upwards and endless varieties of results now seen in the procreative act.-The question is not, can evolution make the mastax of M. ringens? but, has it done so? A great geological professor derived teapot from tepeo, to be warm, but he still left a better derivation possible.

* Since writing the above I have had M. pilula sent me (see 'Science-Gossip,' Jan. 1872, and M. M. J.,' vol. viii. p. 6), and its habits bear on the views expressed in the text. It is well known that whenever M. ringens ejects fæces it bends back the disk dorsally over the anus, as if stooping over the edge of its tube, the anus then protrudes upwards and shoots the refuse up in a cloud of small particles. To anyone acquainted with the animal the object of this attitude is obvious-it is taken to ensure the free passage away of the fæcal matter, and to prevent it running all round the lobes a second time-the result being that as a fact it goes to the ventral side of the main ciliary wreath, and as it were over the shoulder of the animal and then into the central stream of waste over the chin, and then away. Now M. pilula adopts precisely the same course, but with a different result; for whereas M. ringens wants to be rid of its fæces, M. pilula wants to keep them the first consolidates the matter within the lowest part of the alimentary canal into an oval brick, and then ejects it into the stream of waste precisely as does M. ringens; but instead of its passing over the chin, it is there stopped and caught (the chins of the two animals being very similar), and then laid in the attitude given by Mr. Cubitt ( M. M. J.,' vol. viii. p. 5). If the reader will compare this method of brick-making and brick-laying with that pursued by M. ringens (see M. M. J.,' vol. xviii. p. 214), I think he will find some interesting conclusions involved in the comparison.

P

VOL. I.

III.-Note on the Effect produced on P. angulatum and other Test Objects by excluding the Central Dioptric Beam of Light. BY JOHN WARE STEPHENSON, F.R.A.S., Treas. R.M.S.

(Read before the ROYAL MICROSCOPICAL SOCIETY, June 5, 1878.) IN the hope that it may be of interest to the Fellows of the Society, as illustrating, in some measure, the demonstration by Professor Abbe, that the resolution of lined as well as of some other objects depends on the diffraction spectra produced by the structure of the objects themselves, on the light passing through them. I have this evening placed on the table a slide of Pleurosigma angulatum under the new large-angled oil-immersion lens with the centre stopped out, an experiment which also proves, if that be necessary, that in an objective of the best construction the centre is not essential to excellence of definition.

The light passing through the object in the present experiment is transmitted by the achromatic condenser, and is absolutely central. Under ordinary conditions this would show, on taking out the eyepiece and looking down the tube of the instrument, one bright central light from the lamp, with the six equidistant surrounding diffraction spectra, produced by the lines (if indeed lines they be) in the object itself; but at the back of the objective and close to the posterior lens I have placed a stop made of black paper, which entirely excludes the central beam of light; in this stop, however,

six marginal openings have been made, through which the diffraction spectra pass. The result is, that in lieu of the ordinary hexagonal markings, the valve appears of a beautiful blue colour on a black ground, and covered with circular spots, which are very clearly defined with the deepest eye-pieces; in fact, it is now so shown with a deep astronomical eye-piece made by the elder Dollond in the early part of this century.

This appearance is in exact accordance with Professor Abbe's theory. In answer to a letter in which I described the effect produced in the above experiment, Dr. Abbe informed me that the Philosophical Faculty of the University of Jena had proposed as a question to the mathematical students the effect produced in the microscope by these interference phenomena, and he enclosed with his reply a copy of a prize essay written thereon by Dr. Alfred Eichhorn. One problem was that of the

appearance produced by six equidistant spectra in a circle; these correspond precisely with the spectra of Angulatum, and the drawing deduced from theory, as shown in the annexed figure from Dr. Eichhorn's essay, is in exact accordance with that now presented by P. angulatum under the oil-immersion lens; but the smaller markings between the circular spots of the drawing are indicated by faint points only, under the microscope, without any definite shape, and would possibly have escaped observation altogether had not mathematical theory pointed out that such images ought to appear. The great interest in this drawing is, that Dr. Eichhorn had never seen a diatom under the microscope, and gave graphically notwithstanding the purely mathematical result.

If we still further manipulate the spectra by shutting out each alternate spectrum, leaving only three, we come back again to the hexagonal markings; but, as I have previously shown,* they are three times as numerous as under normal conditions, the length of the sides of the equilateral triangle formed by the spectra being respectively as √3:1. Hence we see that we can by regulating the spectra show at pleasure large or small hexagonal markings, circular spots, or even rectangular figures,† the latter form depending on the admission of a spectral image of the second order.

Taking a valve of P. formosum, and simply stopping out the centre of the objective as before, one sees on looking down the tube without the eye-piece, the whole of the back of the lens filled with the spectra arising from the right-angled lines (or dots) of the diatom; but in this case, from the coarseness of the markings, the perfect spectra are admitted, and we have a beautiful white object on a black ground; but instead of the " plate of marbles" the appearance presented is that of circular holes punched out of a silvery plate.

Treating a test scale of Podura in a like manner, i. e. with perfectly central light from the condenser and no dioptric beam, we have a silvery scale with parallel black lines running from one end to the other, with a total disappearance of the exclamation markings. This appearance corresponds to a great extent with that demonstrated by Mr. Beck on one of our scientific evenings, on which occasion condensed moisture from the breath could be seen running down the scale.

It will be remarked that the black ground produced by stopping out the centre of the objective differs essentially from that obtained by a stop in the condenser. In one case the light is stopped after entering the objective, and in the other the great obliquity of the incident ray prevents the lens taking it up.

*Monthly Microscopical Journal,' vol. xvii. p. 88.

+ 'Journal of Royal Microscopical Society,' vol. i. p. 54.