B to form a perfectly correct idea of the shape of any object by a few turns of TO RIGHT EYE BA A FIG. 14. A TO LEFT EYE C the focussing screw. Hence, persons whose means are limited, and who desire to invest a small sum of money.in a microscope to be used for purposes of study, would do well to forego anythought of purchasing binocular stands. On the other hand, there are certain qualities of binoculars which commend them to all workers who can afford the additional cost. Apart 1 FIG. 15. from any stereoscopic effects, it is eyes The stereoscopic effects while not of great practical importance, as already stated, certainly render many objects more attractive to look at. For this reason a microscope for the entertainment and instruction of friends should certainly be a binocular. But it should not be forgotten that the angular aperture of the objective must not be too large, as then the stereoscopic effect will be exaggerated, as Carpenter has well shown. He found that a 1⁄2-inch of 40° gave the true stereoscopic effect. It should be remembered that the ordinary form of binocular cannot be advantageously used with objectives of less than half an inch focus. Therefore, the kind of work to be done should also be considered in selecting the stand. We will now briefly refer to some of the different forms of binoculars, but a detailed account of them cannot be given here. Most of the cuts are taken from the article by Mr. Geo. E. Fell, on "The Binocular Microscope and Stereoscopic Vision," published in the Proceedings of the American Society of Microscopists, and The ordinary Wenham binocular is represented in fig. 18. This is the form in almost universal use in this country. Fig. 19 represents Prof. H. L. Smith's arrangement by which the rays from the objective are partly transmit course can be followed by means of ted and partly reflected by the plate the diagram. Mr. Wenham has devised a prism D. The first binocular of all was an A FIG. 20. A FIG. 21. The (Abbe binocular eye-piece fig. 20), was fully described in this JOURNAL some time ago (Vol. I, p. 201). A similar arrangement was described by Prof. F. A. P. Barnard, represented in fig. 21. The latter, however, was not an ocular, but was intended to be used in the tube. It differs from Prof. Abbe's device, in having a prism of calcite. The Vertical Illuminator. [Dr. E. Van Ermengem recently recounted his experiments with the vertical illuminator before the Belgium Microscopical Society. As this instrument has lately attracted considerable attention in this country, as well as abroad, the following summary from the Bulletin des Séances, may prove of interest.-ED]. This apparatus, invented in 1866, by Prof. Hamilton L. Smith, has been for some time used in a manner different from that designed by its inventor. With the homogeneous-immersion objectives it gives results which have arrested the attention of many microscopists, and which have been the object of interesting discus A sion at one of the late sessions of the Microscopical Society of London. The speaker has made some improvements upon the model of R. & J. Beck, and added an adjustable diaphragm, by means of which the instrument is made more useful and gives better results. Moreover, the reduced dimensions of the apparatus thus modified permits of its use-with Zeiss homogeneous objectives-on microscopes of the smallest model. With a very simple and cheap accessory one may thus resolve the most difficult tests, A. pullucida and the 19th band of Nobert's plate, more perfectly than with immersion condenser, Woodward's prism, swinging substage, etc., which are only applicable to large microscopes. By means of an apparatus constructed by M. De Simpelaere, of Brussels, and a-inch objective of Zeiss, several tests were successfully shown,-Surirella gemma, Navicula rhomboides, Amphipleura pellucida, and the scales of Podura plumbea, were resolved with extraordinary clearness. Under a magnification of 3200 diameters, the striæ of A. pellucida appeared manifestly separated into pearls or Mr. Stephenson seems to have demonstrated that the rays condensed upon the object, at least when it is transparent, as are the silicious valves of the diatoms, are not reflected from the superior surface, and that they traverse it, on the contrary, without illuminating it. Only one portion of the rays, of which the obliquity exceeds the critical angle (41°), suffer total reflection from the inferior surface of the object, and instead of passing into the air, illuminate it by making it selfluminous. Thus is explained why the apparatus only produces this particular illumination with objectives having angle of aperture greater than the maximum of dry-objectives, more than 180°, and on preparations mounted dry having the object adhering to the cover. Dr. Ermengem, moreover, called the attention of the members to the advantage of this method of illumination in the study of certain histological elements. There are many anatomical preparations which can be temporarily mounted dry on the cover-glass. Human blood cells presented an extraordinary appearance, in their bright-red color, their very apdots. The appearance of Pleurosig-preciable relief, and the clearness with ma angulatum under this illumination is very unexpected. The markings resemble the alveolar or hexagonal structure of certain species of Triceratium, Coscinodiscus, etc. This special illumination affords a new confirmation of the opinion held by some microscopists regarding these markings, and which M. Rutot reproduced at the séance of October 30th, 1880, à propos of the microphotographs of this Pleurosigma prepared by Dr. Günther, of Berlin. The The Limiting Diaphragm, or For many years past we have heard declamations against the practical principally, however, upon the ground value of wide aperture objectives, that they do not possess penetration, though all seem agreed that they do define much better than those of small or medium angle. Even Dr. Carpenter, the champion of medium angles, in the last edition of his work, P. 732, seems at last to prefer wide apertures, for in the only instance in which he has expressed an opinion upon the structure of insect scales from his own study he writes: "The appearance of solidity, of relief, afforded by objects thus illuminated, led effects are due to an illumination very * From The Northern Microscopist. "author has fully satisfied himself by "his own study under an oil immer66 sion of Messrs. Powell and Lea"land of a Podura scale illuminated "by the 'immersion paraboloid,'' etc., etc. It is well known that the wider the aperture of the objective, the less the working-distance may be, though this does not depend upon the aperture alone. A reference to pp. 257, 282 and 283 of this journal will show this clearly, if columns a, d and g are compared; we do not, however, wish to enter into this question at present, but will endeavor to show how penetration may be given to wide angle lenses. It seems upon inquiry that some of our opticians have for some time past produced a half-inch objective for use with the binocular, by placing a diaphragm of smaller aperture than. usual behind the back lens of the objective. The half inch of 60° is thus easily reduced to 40°, and the penetration consequent upon such reduction is by this means obtained. For some time past we had been using stops of blackened card-board, and as these were not very convenient we had a conversation with Mr. J. B. Dancer of Manchester, as to the utility of making an "iris" diaphragm for the purpose, when he produced a graduating diaphragm made ten years ago, but to the best of our knowledge, our idea has never been published. The It will be seen that this form of aperture shutter enables the operator to adjust his objective to any aperture he wishes, and this cannot be effected upon any older plan without having a large set of stops on hand. closing of the shutter does not contract the absolute size of the field, but only the brightness of it, and the true value of penetration can easily be observed without moving the eye from the tube. The value of wide apertures for good definition may also be seen when using this "shutter." If Topping's admirable preparation of the proboscis. of the blow-fly be observed with an inch objective having an air angle of 30°, the view is superb, the pseudotracheal markings come out well defined and sharp; but close the shutter until an angle of 14° or less is obtained and examine again, when it will be found that the definition is not nearly so good, while there is more penetration, the whole of the pseudo-tracheal tube being observed under one focussing. While in this condition, the eye being still applied to the tube, open the shutter to its full extent, and the effect of wide aperture will demonstrate itself. Perhaps the best object to show the amount of penetration possessed by objectives of low angle, may be found in the microfungus Myxotrichum deflexum, or M. chartarum observed under the one-inch objective. The former object consists of little patches of grey downy balls, from which arise a number of radiating threads furnished with a few opposite and deflexed branches. Under an inch objective of 30° air angle but few of these branches can be seen under one focussing, the remainder being enveloped in a haze of light; but if a central layer be focussed, the simple closing of the shutter will suffice to bring the superior and inferior layers into view, though of course the image is not so bright or so well defined as before. The objection to wide angles, that they do not possess penetration may now be fairly said to have broken down; its other phase, that of working-distance, we will treat of in a future number. Staining of Living Unicellular Organisms.* K. Brandt finds hæmatoxylin and Bismarck-brown suitable coloring materials with which to stain Protozoa in the living state. For Amabæ and Heliozoa a dilute solution of hæma *Abstract, in Journ. Royal Micros. Society. |