On a microscope adapted for Anatomical Demonstrations ; and on a Binocular Microscope

These difficulties are experienced still more in the demonstration of microscopical dissections ; for the observer, to avoid losing considerable time, exhibits only the result of his operations, and not the method by which he arrived at it, though that is also very important. The problem then has been, how to enable two or more persons to observe at one time the same object. I believe I have resolved this difficulty by the construction of the instrument represented in fig. 1, with which two persons looking through the eye-pieces, C C′, obtain each an erected image of the object ; the division of the pencil of light formed by the objective, A, is accomplished by a prism, P, fig. 2, the section of which is an equilateral triangle ; the image reflected upon the face a emerges normally upon the face b, that reflected upon b also emerges upon the face a ; there is thus formed on the right and left of the instrument an image erected in a certain sense ; if upon the transit of these rays the prisms, B B′ fig. 1, are placed, similarly to the separating prism, but so disposed that their reflecting surfaces are perpendicular to the central prism, the object will then be completely erected on each side, which is a considerable advantage for the demonstrating of dissections ; moreover, the two observers can place themselves on the same side of a table, for the eye-pieces are brought nearly into the same plane as regards the prisms B B′, and are placed at a very convenient inclination for avoiding fatigue to the head. If the two have not the same focus, one is first adjusted by the pinions and screws, and the other by drawing out and pushing in the tube carrying the oculars. If a vertical view be desired, the screw-heads D D′ have only to be turned and the prisms B B′ to be revolved a quarter round, to obtain an effect exactly opposite to the preceding one, that is to say, that the images are reversed as in an ordinary microscope, for the faces of B B′, fig. 1, becoming parallel to those of the central prism, destroy the first erection caused by it.

It will be seen that by leaving B inclined and by bringing B′ into the perpendicular, a person looking in C will have an erected image, whilst another looking vertically in C′ will have a reversed image.

The parallelism of the oculars C C′, when they are in a vertical plane, brings to mind Professor Riddell’s valuable method of simplifying the construction of binocular microscopes, for if the prisms B B′ are brought towards the central prism, as in fig. 3, the oculars C C′ are placed so as nearly to suit the distance between the eyes.

Fig. 4 shows sufficiently well the course of the rays, and requires no further explanation ; I shall only remark that the rays, a b a′ b′, always emergingnormally on the terminal faces, there is no chromatic dispersion ; and the superior angle of the prism being very acute, and the surfaces perfectly even (which is not difficult of execution), the loss of light is almost inappreciable. There is a great advantage attached to this arrangement in the impossibility of pseudoscopic effects arising, as in Professor Riddell’s last and definite arrangement, or in Mr. Wenham’s very ingenious refracting prism. When the distance between the oculars requires to be modified, the screws V V′ are used for moving the lateral prisms nearer or further off at pleasure.

The phenomena of complementary colours produced by polarization can be examined with this instrument in the most simple manner ; and with regard to crystals, it is certainly one of the most beautiful spectacles to behold a crystal in relief, appearing tinged with complementary colour to the single eye, while to binocular vision it becomes white, as if it were not seen under polarized light.