ZOOLOGY AND BOTANY, MICROSCOPY, ETC. <S17 



all sufficiently transparent to be taken by transmitted light without any 

 previous preparation. One of the chief obstacles to success in stereo 

 work with the Microscope, is the small depth of focus given by micro- 

 scopic objectives. If the specimen has considerable depth of structure, 

 the lowest power possible must be used, and the necessary magnification 

 obtained by the eye-piece. WTien I ransparent objects are photographed, a 

 narrow angle of illumination should be adopted. This will decrease re- 

 solution, but increase depth of focus. When a specimen has strong con- 

 trasts in light and shade, it is sometimes advisable to give a normal 

 exposure for the first negative, and then over-expose the second plate, in 

 order to get detail in the darker portions of the subject. A much 1 tetter 

 effect is given by this plan, when the two prints are viewed through the 

 stereoscope. In the same manner, if a double-stained specimen is being 

 photographed, the first negative should be exposed for the deeper colour, 

 and the second for the lighter shade. Generally speaking, for stereo- 

 scopic work, soft negatives give the best effects, and no pure whites should 

 appear in the finished print. The negatives should be printed on smooth 

 paper, such as Ilford Glossy Gaslight. 



(5) Microscopical Optics and Manipulation. 



Pupil of an Optical System with regard to Perspective.* — C. 

 Beck has found that the so-called " entrance and exit pupils " of an 

 optical system may be used, in connexion with the Gauss planes, for ex- 

 plaining the action of optical instruments as regards the perspective of 

 the images formed. It will be remembered that the Gauss planes enable 

 us to refer the action of a complicated optical system to an equivalent 

 single lens placed successively in two positions — the entrance equivalent- 

 plane and the exit equivalent-plane. By this means, assuming that the 

 optical system is corrected in such a manner that the oblique rays and 

 those far from the axis act in the same manner as the direct axial rays, 

 the position and size of images can be determined with accuracy ; but 

 the perspective of the image cannot be correctly explained by aid of the 

 Gauss planes alone. Th°. " pupils," however, account for this apparent 

 discrepancy, and, by assigning the correct position to these two apertures, 

 or pupils, we can investigate the perspective of an image without taking 

 further consideration of the system itself : just as, by assigning the correct 

 position to the two Gauss planes, we can investigate the size and position 

 of the images, irrespective of the system itself. The pupils, in fact, 

 determine what rays form the image, and they further determine the 

 perspective without invalidating the results given by the Gauss system as 

 to the positions and sizes of the focused images. The author discusses 

 in detail several typical cases, and, in particular, clears up a very interest- 

 ing point in the practical use of telephotographic lenses. Such lenses 

 have usually very small apertures, and possess a large degree of so-called 

 depth of focus, and are consequently capable of depicting a great range 

 of depth in the object. For distant views the perspective will, on the 

 whole, give the effect produced by photographing with an ordinary Lens. 

 of about the same focal length as the equivalent focal length of the tele- 

 photo system. But if a telephoto lens be used for near objects, as, for 



* Proc. Roy. Soc, Series A, lxxxv. (1911) pp. 462-70 (8 figs.). 



