ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 691 



of aperture. With low powers, which allow of a relatively consider- 

 able depth-perspective, the slight diiference of inclination, which re- 

 mains in the latter case, is quite sufficient to produce a very marked 

 difference in the perspective of the successive layers in the images. 

 But with high powers the difference in the two images does not keep 

 pace, even when both eye-pieces are half covered, with the increase of 

 the angle of aperture, so long as ordinary central illumination is used. 

 For in this case the incident pencil does not fill the whole of the open- 

 ing of the objective, but only a relatively small central part, which as 

 a rule does not embrace more than about 40^ of angle, and in most 

 cases cannot embrace more without the clearness of the microscopic 

 image being affected, and the focal depth also being unnecessarily de- 

 creased. But as those parts of the preparation which especially 

 allow of solid conception are always formed by direct transmitted 

 rays in observation with transmitted light, it follows that under these 

 circumstances the difference of the two images is founded, not on the 

 whole aperture-angle of the objective, but on the much smaller angle 

 of the incident and directly transmitted pencils, which only allow of 

 relatively small differences of inclination of the image-forming rays 

 to the preparation. It is evident, however, that when objectives of 

 short focus and correspondingly large angle are used, a considerably 

 greater differentiation of the two images with respect to parallax can be 

 produced, if, in place of one axial illuminating pencil, two pencils are 

 used oppositely inclined to the axis, in such a way that each of the 

 images is produced by one of the pencils. This kind of double illu- 

 mination, though it cannot be obtained by 

 the simple mirror, can be easily produced "^- 



by using with the condenser a diaphragm 

 with ttoo openings (Fig. 160), placed in 

 the diaphragm stage (or carrier) under 

 the condenser. We then have it in our 

 power to use at pleasure pencils of nar- 

 rower or -wider aperture and of greater 

 or less inclination towards the axis, by 

 making the openings of different width 

 and different distance apart. With dia- 

 phragms of this form (which can easily 

 be made out of card), the larger aperture 

 angles of high-power objectives may be made use of to intensify the 

 stereoscopic effect without employing wide pencils, which are preju- 

 dicial, both as diminishing the clearness of the image and the focal 

 depth. Of course, with this method of illumina- 

 tion both eye-pieces * must be half covered, in order Fig. 161. 

 that one image may receive light only from one of i i 

 the two illuminating cones, and the other only from ^"^ (^ 

 the other. The division of light in both the aper- I 

 ture-images will then be as shown in Fig. 161, and 

 it is evident that in this case the brightness of the image for both eyes 

 together is exactly the same as would be given by one of the two cones 

 alone without any covering. 



* That is, of the Abbe Stereoscopic Eye-piece. 



