104 



VISION WITH THE COMPOUND MICROSCOPE 



plane of the |iri'ii;ii-:ili(Hi. Mini tin- deilected image by one whose axis 

 is inclined about a fourth of the angle of aperture. 



\\'ith low powers, which allo\\ of a relatively considerable 

 depth-perspective, tin- slight difference of inclination, which remains 



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 in- 

 crease 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 

 opening of the objective, but only a 

 relatively small central part, which, 

 as a rule, does not embrace more than 

 40 of angle, and in most cases can- 

 not embrace more without the clear- 

 ness of the microscopic image being 

 affected and the focal depth also being 

 unnecessarily decreased. But as 

 those parts of the preparation which 

 especially allow of solid conception 

 are always formed by direct trans- 

 mitted rays in observation with trans- 

 mitted light, it follows that under 

 these circumstances the difference of 

 the two images is founded, not on the 

 whole aperture-angle of the objec- 



make the path of the rays clearer, live, but on the much smaller angle 



of the incident and directly trans- 

 mitted pencils, which only allou- of relatively small differences 

 of inclination of the image-forming rays to the preparation. It is 



exident, houever. that when objectives 

 of short locus and correspondingly large 

 angle an- used, a considerably greater 

 diiferentiation of the two images with re- 

 spect to parallax can lie 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 l>\ one of the pencils. 

 This kind of double illumination, though 

 it cannot he obtained by the simple 

 mirror, can he easily produced by using 

 with the condenser a diaphragm with two 

 -'). l'].-"vd in the diaphragm stage under the coii- 

 We then have it in our power to use, at pleasure, pencils 

 nai> rower or \\ider aperture and of greater or less inclination 



FIG. 81. Simpler illustration of tin- 

 path of the ray with one prism 

 turned through an angle <>t' '.MI t,n 



FIG. 82. 



o! 



