III.—On a New Operculated Infusorian from New Zealand. By F. W. HUTTON, Professor of Zoology in the University of Otago.

(Read before the ROYAL MICROSCOPICAL SOCIETY, March 6, 1878.) In the Monthly Microscopical Journal' for 1869, vol. i. p. 289, Mr. W. S. Kent described, under the name of Cothurnia operculigera, an infusorian bearing an operculum. Last November, in a fresh-water lagoon near Dunedin, I found a very similar form in considerable abundance. The New Zealand species, however, differs from C. operculigera in having the pedicle much shorter than the lorica, and in the aperture of the lorica being oblique.

RUFFLE

Cothurnia furcifer. 400. a, operculum.

In adult specimens the lorica is of a deep chestnut brown, and opaque; it is generally more or less crumpled, but sometimes smooth. The aperture is round, entie, and oblique to the axis of the lorica. The average length of the lorica is ". The operculum (a) is circular, the same size as the aperture, and of the same colour as the lorica; it is attached to the animal just below the peristome, on that side towards which the aperture slopes. The pedicle is transparent, ve y short (one-sixth the length of the lorica), or occasionally absent; it is attached at its proximal end to the conferva, on which it lives, by a circular disk of about the diameter of the lorica, and of the same brown colour. The animal is colourless, considerably smaller than the lorica, and attached at the base without any stalk. The contractile vesicle is central. The peristome is surrounded by a moderate number of

rather long cilia, which lie nearly horizontally when the animal is fully expanded. Inside these are four much more robust cilia, which are once divided, like a hay-fork. These stand more upright; two are situated just above the operculum, and two on the other side of the peristome. Both these sets of cilia twitch spasmodically at uncertain intervals, but have no uniform motion. Lying internally to these are a set of very short cilia, which are constantly active both when the animal is protruded and when it is withdrawn into its sheath.

In the young the lorica is nearly transparent; and there is no operculum until the lorica is fully shaped out. It then commences to grow, but does not get the full size of the aperture until the lorica has become quite dark coloured. I have never seen two in a lorica.

I propose to call this species Cothurnia furcifer, from its four fork-like cilia. The likeness of this infusorian to the capsule of a moss is remarkable.

IV. On a Large-angled Immersion Objective, without Adjustment Collar; with some Observations on "Numerical Aperture." By JOHN WARE STEPHENSON, F.R.A.S., "and Treasurer of the Royal Microscopical Society.

(Read before the ROYAL MICROSCOPICAL SOCIETY, April 3, 1878.) IN the use of the microscope few things are perhaps more difficult than the effective application of the adjustment collar of a largeangled modern objective.

Not only is the best point of correction difficult to attain, but in too many cases, either because the covering glass is too thick or too thin, the slide is not suitable, and it is impossible to do justice to the probably otherwise excellent quality of the lens.

In fact, so indeterminate is the best point, that an objective thrown out of adjustment would seldom be brought back to the identical number on the scale, if readjusted, by any ordinary observer, even on a well-known object.

If this be so, on an object with which one is familiar, how much more difficult must be the examination of an object the structure of which is perhaps wholly unknown.

Such considerations suggested to the writer the great desirability of constructing an object-glass in which cover correction could be entirely dispensed with.

The origin of the correction collar as shown by the late Andrew Ross, was the imperative necessity of compensating the error arising from the difference between the refractive index of the covering glass and that of the air between the front of the lens and the thin cover, whenever a high power was used.

Now it is evident that if some fluid, of which the refractive and dispersive powers are the same as those of the covering glass, were substituted for air in the intervening space, the end in view would be attained.

If, then, such a medium could be found, with, as I said before, optical properties identical with those of the thin cover, it follows that as the distance between the front lens of the objective and the under side of the cover (or the object adhering to it) must, of necessity, be always the same; that is to say, as a thick covering glass would require a thin film of the proposed immersion fluid, and conversely as a thin cover would require a thicker layer (one, in fact, being the complement of the other); the two together would have a constant value, which would be exactly equal to the focal length of the objective.

From this point of view the writer suggested to Professor Abbe the possibility and desirability of the construction of a combination satisfying the conditions named, and that gentleman with

characteristic energy at once applied his great knowledge to the solution of the problem, and with complete success.

He informed me that he had already from another point of view considered the propriety of using a more highly refractive medium than water for "petrographic" work (Dünnschliffe), as the thin plates of minerals ordinarily used for microscopic inspection are generally so roughly cut and insufficiently polished that their observation with high powers is very difficult, if not impossible.

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The more general problem of constructing an objective in which the necessity for correction, by an alteration of the distance between the lenses, could be entirely dispensed with, was the more attractive to the Professor as the use of a more refractive medium than water enabled him to deal with higher apertures, without loss of definition, than had hitherto been attained. The result of his calculations was the construction by Mr. Zeiss of an objective of excellent quality, satisfying the conditions named and having a balsam angle of 113° 125 of " numerical aperture"; an aperture which is, if I mistake not, larger than any that has been hitherto produced, and one which coincided exactly with the theoretical value given by Professor Abbe's formula. But here an unexpected difficulty arose: it had been assumed by Professor Abbe and myself that amongst the numerous fluids suitable for use, there would readily be found some, pure or mixed, which would give exactly the same refraction and dispersion as crown glass; this, however, proved not to be the case, and the difficulty was not overcome until he had tested no less than sixty-three different oils, and nearly thirty other fluids: he at length ascertained that oil of cedarwood, although not absolutely identical with crown glass, was admirably adapted for the purpose, giving perfect definition with oblique light, but, for central light, being improved by the addition of one-fourth or onefifth of oil of fennel seed (Ol. Foeniculi). With central light the difference between the spherical aberration of the green and red rays gave, when pure cedarwood oil was used, too much colour, which was corrected by the addition of a small quantity of the more highly dispersive fluid. In the course of these numerous experiments it was found that the tables given by Brewster and Wollaston were very unreliable, nearly all the refractive indices given by those authors being considerably too high, and most of the dispersion values too low. As it is probable that further research may still result in finding a more perfect medium than that now used, I may mention that Professor Abbe determines the suitability of a medium by using a bottle, the sides of which are of parallel plate glass, and to whose stopper a small prism of crown

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The asphalte varnish on the slides can be protected from the dissolving action of the oil by a coating of sealing-wax varnish or gold size.

glass has been fixed. By plunging the prism into the fluid, the refractive and dispersive properties of any oil or mixture of oils can be determined and adjusted by simply viewing the bar of a window through the liquid and the prism.

The qualities of the new objective may be thus briefly described :

1. There being no aberration to correct for varying thickness of cover-glasses, there is no collar adjustment. For thick covers, say 0.008 to 0-009 the ordinary length of 10 inches gives the most perfect definition; for thin covers, say 0.004, a length of 12 inches is perhaps better. But the difference is so very slight that it is scarcely necessary to use the draw-tube.

2. It has a balsam angle of 113° = 1.25 numerical aperture, which renders it extremely sensitive in focussing, and also indicates the highest resolving power hitherto attained.

3. It has a large working distance. The distance between front of lens and object is 0.02, which gives a working distance of 0.012 for 0.008 cover-glass, 0·016 for 0.004 and so on.

4. Its power is rather more than one-ninth, and having component lenses throughout the combination, larger than in other objectives of the same power, it transmits more light, the latter quality being enhanced by a diminution in the reflection of the peripheral rays.

5. It bears very deep eye-pieces and has a flat field.

Lastly. An essential condition to its perfect performance is that if the object be dry, it must be mounted on, or nearly touch, the cover, or if not a dry object, that it be mounted in some medium having approximately the same refractive index as the oil, such as Canada balsam, &c.

The special advantage of this objective for petrographic work is, that the oil used, having very nearly the refractive index of the objects to be examined, renders cover-glasses and highly polished surfaces unnecessary; the minerals, if sufficiently translucent, can be observed through a considerable depth, say of an inch, and, on the assumption of identity of index, every plane, from the surface downwards, will have the same perfect correction.

To a certain extent the latter observation applies to all transparent objects mounted in balsam, as the thickness of the balsam above the object may be looked upon as equivalent to a thicker cover, and the penetration must therefore be considerable, although the latter quality will be still greater in the smaller-angled objectives constructed on the same principle, which will hereafter be made.

Professor Abbe, writing to me on the objective, says: "The advantage of the greater aperture is shown in a most striking manner on Pleurosigma angulatum, when observed by very oblique