AMERICAN THE NATURALIST. Vol. VI.-APRIL, 1872. - No. 4. مد THE USE OF AMPHIPLEURA PELLUCIDA AS A TEST-OBJECT FOR HIGH POWERS. BY DR. J. J. WOODWARD. U. S. A. OVER a year ago (February 1, 1871) the Surgeon General of the United States Army published a brief memorandum prepared by me, on the Amphipleura pellucida and its markings. This memorandum was accompanied by two photographs exhibiting the striæ of the diatom, as seen with a power of about one thousand diameters. The paper was republished in the "American Journal of Science and Arts" (May, 1871), and in the "London Monthly Microscopical Journal" (July 1871). Since preparing the memorandum referred to, I have had occasion to use the Amphipleura pellucida a number of times as one. of the means of comparing the high power objectives of various makers, and having found it, within certain limits, well adapted to this purpose, have thought the following remarks on its use would not be without interest to working microscopists. Specimens mounted by various English preparers may readily be obtained from any of the large dealers in microscopical preparations. I have compared such modern slides with some of the original ones mounted by Messrs. Sollitt and Harrison, which I owe to the courtesy of Mr. W. S. Sullivant of Columbus, Ohio, with the sample in the first century of Eulenstein, and with other slides from various sources. I find all very much alike, the stria usu-. ally varying from ninety to one hundred to the Too of an inch. In a few large frustules I have found coarser striæ than the above Entered according to Act of Congress, in the year 1872, by the PEABODY ACADEMY OF SCIENCE, in the Office of the Librarian of Congress, at Washington. AMER. NATURALIST, VOL. VI. 13 (193) but finer ones in none. For the best use of the test it is essential that the frustules should be clean and mounted dry,* on the under surface of a very thin cover (not thicker than zʊ of an inch). In some of my slides the frustules are mounted between two thin covers, adhering to the upper one but I am not sure that this arrangement offers any decided advantages. The first step in the practical use of this test, after obtaining a properly mounted specimen, is to select a frustule, to count the number of its striæ to the thousandth of an inch, and to record its position with a Maltwood's finder. The frustule thus selected becomes a valuable unit of comparison between different objectives, the distinctness with which the striæ are shown indicating the definition of the glass, the manner in which the edges of the frustule are seen while the mid-rib and striæ are in focus showing the degree of penetration, and the appearance of the ends of the frustule when the centre is in focus giving a fair idea of the flatness of the field. The illumination must be oblique, and the pencil of light must be thrown lengthwise along the frustule, which may be done by a common coal-oil lamp, with or without a small plano-convex lens, or other condensing apparatus, to concentrate the rays. This, however, is the least favorable mode of illumination, and will only succeed if very carefully used with the best objectives. Much better are the calcium and magnesium lamps, which may be condensed obliquely by means of a small plano-convex lens of one to three inches focal length. Either source of light gives a beautiful picture, the striæ being black on a white ground. The finest *This is essential to the best and most beautiful appearance; it is not, however, indispensable for resolution, nor does balsam mounting make resolution much more difficult. For example I obtain excellent resolution of the balsam mounted Amphipleura pellucida on the Museum Möller's type-plate by Beck's immersion 1-10; price £6 sterling. The stria are distinct and easily counted but paler than on dry specimens. I desire, also, to draw attention to the fact that Count F. Castracane in a paper read before the Royal Microscopical Society, March 1, 1871, expressly states that the year before he had made a photograph of the balsam-mounted Amphipleura pellucida of Möller's type-plate, obtaining good resolution and counting the stria, which he gives as 4,000 to the millimetre. For this purpose he used a No. 10 of Hartnack, illuminated with monochromatic sunlight obtained by a prism. His negative" was blurred and rather faintish, so that it would not give good positive images." The power was about 640 diameters. Afterwards he obtained the same results with a No. 10 of Nachet (Monthly Microscopical Journal, April, 1871, p. 176). I may add that the Hartnack's No. 11 belonging to the Museum (price 250 francs) gives excellent resolution of the Amphipleura of Möller's type-plate and of other balsam-mounted specimens belonging to our collection. results, however, are attained by the light of the electric lamp or of the sun rendered monochromatic by passing through a saturated solution of the sulphate of copper in strong aqua ammoniæ and of about the eighth of an inch in thickness. Of these methods, that by sunlight involves least trouble and expense, and may be best managed as follows:-Erect a perpendicular wooden screen about two feet square on one edge of a small table. Cut in this a circular hole an inch and a half in diameter at about the height of the under surface of the stage of the microscope. On the outside of this hole mount a small plane mirror which can be adjusted by passing the hand to the outside of the screen. On the inside, cover the hole with the ammoniosulphate cell. (A piece of dark blue glass will answer the purpose though not so well.) Now move the table to a window through which the direct rays of the sun can fall upon the mirror, adjust this so as to throw a nearly horizontal pencil of parallel rays through the hole, and place the microscope in the shade of the screen in such a position that the parallel blue rays will fall on the under surface of the amphipleura slide at an angle of from fifty to seventy-five degrees with the plane of the slide (I suppose the frustule to be examined has first been found by ordinary day-light or lamp-light). Next place a small bull's eye or any other condenser of from one to three inches focal length (mounted on a separate stand or on a radial arm) in the parallel pencil in such a position as to concentrate the light, at the angle above indicated, upon the frustule under examination. After this nothing remains but to regulate the cover correction and the fine adjustment. The precise angle which should be given to the illuminating pencil will vary with the angle of aperture of the objective used. As a rule it should be less than half the angle of aperture of the objective, and 70° to 75° is the maximum angle which should be given even for objectives of 170° angle, a greater angle, distorting the image without improving the definition. The same results can be obtained by using a heliostat to fix the direction of the solar rays, and obtaining obliquity by an achromatic condenser of from 130° to 150° suitably decentred. On account of the stability of the illumination this method is especially suitable for photographing the Amphipleura, but the simpler method above described answers every purpose if the object is to compare objectives. |