being straight as in G. marina. I have been unable to obtain a s. v. of the diatom. I, however, believe them to be arcuate.

RHABDONEMA.

18. R. hamuliferum (F. Kitton). Valves lanceolate, undulate, with transverse punctate striæ, ends rounded smooth, diaphragms* striate their whole length, having in their centre a circular ring, and towards the extremities two others much smaller and elliptical. Seen in f. v. the frustules appear composed of a variable number of diaphragms (annuli) placed between the two valves, and separated the one from the other by lines in the form of hooks; each of the diaphragms is striated on the edge. Length 33 to 78 μ, breadth 11 to 15

μ

On corallines, Lyell's Bay. Pl. XV., Fig. 22.

This form is distinguishable from all other species of Rhabdonema by the undulations of the margin of the valves, and by the lines separating the diaphragms being bent in the form of hooks. Mr. Kitton recognized it as new, and named it hamuliferum.

AULISCUS, Ehr.

19. A. stelliger (P. Petit). Valves circular, with four processes, two of which are very small. The valve is divided into three concentric parts, the innermost has five stellate rays, the intermediate portion has the rays irregularly disposed, the external circle has a row of puncta on the inner margin corresponding with the rays of the intermediate part; in this outer ring are situated the four processes, between which are irregular rays. Diameter 35 μ 2.

Campbell Island (rare). Pl. XV., Fig. 25.

This species is distinguished from all other Aulisci, recent or fossil, by the peculiarity of the disk and by the presence of the interrupted rays.

The following genera and species of other authors are figured by M. Petit:

1. Cocconeis Wrightii (O'Meara), 'Q. M. J.,' vol. vii. n. ser. p. 246, pl. vii. fig. 6.

Lyell's Bay. Pl. XIV., Fig. 3.

The figure here given represents a very small form, not exceeding 13 μ to 26 μ 4, but the difference from the type form is too little to allow of its being considered a variety; by altering the focus of the objective a dilated cruciform nodule may be detected.

* The so-called diaphragms as figured by M. Petit are valves; the diaphragms (annuli) in this genus are only striate on the edges. The "hooks" represent the septa as scen in s. v.

2. C. Crux (Ehr.), 'Monatsberichten,' p. 265.
Lyell's Bay. Pl. XIV., Fig. 4.

3. Campyloneis Grevillii (Wm. Sm.).

Since the creation of the genus by Herr Grunow, he has discovered that C. Grevillii consists of two valves; the inferior is composed of two superimposed plates of silica, of which no drawing has previously been given. Fig. a represents a complete frustule; Fig. d, the superior valve; b and c, the two plates of silica composing the inferior valve.

4. Navicula decussata? Ehr. Bericht. Berl. Akad.' p. 364. Length 30 to 39 μ, breadth 7 μ 7.

Lyell's Bay. Pl. XIV., Fig. 11.

This species with crossed striæ has not been previously figured; I therefore doubtfully refer the present form to this species. Nevertheless all the characters correspond with those given by Ehrenberg.

5. N. Hennedyi (Wm. Sm.), var. B constricta.

Foveaux Strait. Pl. XIV., Fig. 13.

This form has probably not been figured; it is chiefly remarkable on account of its occurrence with a species essentially elliptical, clearly showing that the contraction of the margins is of no value as a specific distinction.

6. N. Smithii (Breb.), var. ẞ minor (P. Petit), n. var.

This variety is distinguished by its broadly rounded ends, and its finely punctate striæ. Length 33 μ, breadth 19 μ 8, costa 15 to 16 in 15 μ.

Lyell's Bay. Pl. XIV., Fig. 14.

μ

7. Grammatophora marina (Lyng., n. var. (P. Petit).

This variety differs from the type, by the lines of the diaphragms being more bent.

Foveaux Strait. Pl. XV., Fig. 20.

New Diatoms from Honduras. By Herr GRUNOW.

Monthly Microscopical Journal,' October 1, 1877.

ERRATA.-PL. CXCIII., for 11 a b c d, read 10 abcd; for 10 a b c, read 11 a b c.

III. Further Remarks on a "Simple Device" for the Illumination of Balsam-mounted Objects for Examination with Immersion Objectives whose Balsam Angle is 90° or upwards. By J. J. WOODWARD, Surgeon and Brevet Lieut.-Col. U.S. Army.

(Read before the ROYAL MICROSCOPICAL SOCIETY, October 9, 1878.) SINCE the publication of the original paper in the 'Monthly Microscopical Journal' for August, 1877 (p. 61), I have been in the habit of using a similar prism to that there described, without any diaphragm, in the ordinary lamplight illumination of balsammounted tests for study with objectives of the kind referred to. The apparatus is so simple, so easily manipulated, and so effective, that a brief description may be of interest.

a

A right-angled prism of crown glass (a), the long side of which is three-quarters of an inch long by half an inch wide, has its right angle truncated as in my "simple device," and cemented to a small base-piece of brass (b) which is supported in a stiff steel rod (c) three or four inches long, and about the thickness of a knitting needle. The whole apparatus ought not to cost more than three or four shillings.

To use it, I slip the steel rod into the holder of the rod of the dark well usually employed with the Lieberkühn mirrors. This holder fits into the sub-stage of the microscope, and putting a drop of oil of cloves on the upper face of the prism, it is racked up into optical contact with the lower surface of the slide. I then condense the light of a small coal-oil lamp upon the object, through either face of the prism, by a small bull's-eye lens of about three inches focal length. One half of this condensing lens may be covered with black paper, in which case it can readily be so placed that no ray forming a smaller angle with the optical axis than 45° can enter the objective. Excellent results can be obtained, however, without this precaution, if the intention be merely to resolve a given test, without caring to know precisely the angle at which this is done; in this case it is only necessary to place the lamp in such a position that a line drawn from its flame to the face of the objective shall be perpendicular to either short side of the prism, and condense the fight in that line. By looking through the other short side of the

prism the observer can readily see when the face of the objective is best illuminated.

For the illumination of immersion objectives which do not much exceed 82° balsam angle, I sometimes also use in the same way a prism of 98° angle, and get thus very good results. I have sent one of each of these prisms to Mr. Mayall, with the request that he will exhibit them to those interested.

Prisms of various other angles may be used, and illumination of any desired obliquity thus secured. It may be laid down as a principle that such delicate striæ as those of Amphipleura pellucida will be most strikingly separated by any given objective, when the most oblique pencil it can admit without distortion of the image is used. But, practically, I have found as yet no objectives which bear, without distortion of the image, a much greater angle of illumination than can be obtained with the 90° prism, and for objectives of lower balsam angle than 90°, but greater than 82°, the 98° prism answers quite as well as one of more nicely adjusted intermediate angle. Immersion objectives of still lower balsam angle than 82°, which cannot be illuminated even with this prism, may be rejected as unworthy serious consideration at the present day.

The two prisms just described will be found, then, to answer all practical uses with our present objectives, and I think it will probably be cheaper to have two separate prisms than to grind a single one, the two short sides of which should be respectively at angles of 45° and 49° with the optical axis, which would of course answer the same purpose.

In this connection, I may remark that the general recognition, in the United States, of the great angles obtainable for immersion objectives, several years ago suggested the importance of immersion condensers to our instrument-makers. Mr. R. B. Tolles wrote in 1871 to the Monthly Microscopical Journal,'* at the close of an article on the aperture question, " Certainly the use of immersion condensers is abundantly indicated in the above simple experiments," and, acting on this indication, he has manufactured a variety of plano-convex, nearly hemispherical, lenses, semicylinders, &c., to be placed, with immersion contact, beneath the slide for this purpose.

Besides these devices and various experiments with immersion objectives placed beneath the stage in lieu of the ordinary achromatic condenser, considerable popularity has been enjoyed in the United States by Wenham's reflex illuminator, which its distinguished inventor designed to give black-ground illumination for objects mounted dry,† but which Mr. Samuel Wells, of Boston, * Vol. vi. p. 36.

† Same Journal, vol. vii., 1872, p. 236.

*

found, in 1875, gave a bright field when used on balsam-mounted objects, provided they were examined with immersion objectives of sufficient angle. Mr. Wells gave an account of this method in the Boston Journal of Chemistry,' in which he says:-" In examining Möller's Probe-Platte, a balsam mount, under these conditions, with light from a kerosene hand-lamp, I easily resolved the Amphipleura pellucida, so clear and decided were the lines that with a power of 8000 they were still visible. . . . The resolution of this difficult diatom, as well as the Frustulia Saxonica and Nitzschia curvula (Nos. 18 and 19 on the Probe-Platte), far surpasses any that I have ever seen by artificial light, and rivals the beautiful resolution obtained by monochromatic sunlight." The paper of Mr. Wells was copied or abstracted in several of our journals, and the method has ever since been in constant use by a number of our microscopists. I note that it has just been rediscovered by Mr. Adolf Schulze, of Glasgow, † who, although apparently unacquainted with the American publication of this method, has arrived at results identical with those of Mr. Wells, as, indeed, all will do who patiently try it with suitable objectives.

I do not claim for my own device, figured in this paper, that it gives better results than the best that can be obtained in this way, or with a suitable hemispherical lens or semi-cylinder, brought into immersion contact beneath the balsam-mounted object; but it is not only cheaper than any of the plans hitherto described, but, as I think, and as all to whom I have shown it seem to find, it is much easier to use so as to get the best results with a fully illuminated field.

In conclusion, I may remark that while it is clearly necessary to use immersion condensers to secure the greatest obliquity of illumination that can be admitted by the widest-angled modern immersion objectives, and while this great obliquity is of substantial advantage in the resolution of lined test-objects mounted in balsam, it by no means follows that such condensers are necessary to the advantageous use of immersion objectives of more than 82° or even of 90° balsam angle, with central light, provided the object is mounted in Canada balsam. The minute details of thin sections of normal and pathological tissues thus mounted and illuminated, are far better displayed by such objectives than by those of inferior angle; and this easily observed fact is so fully in accord with elementary optical theory that its discussion in this place seems quite unnecessary.

*June, 1875, p. 140.

"An Easy and Simple Method of Resolving the Finest-lined Balsamed Diatomaceous Tests by transmitted Lamplight, with special reference to Amphipleura pellucida," Journal of the Royal Mic. Soc.,' May, 1878, p. 45.