104 



VISION' WITH THE COMPOUND MICROSCOPE 



plane of the preparation, and the deflected image by one whose axis 



is inclined about a fourth of the angle of aperture. 



With low powers, which allow of a relatively considerable 



depth-perspective, the slight difference of inclination, which remain.-- 



in the latter case, is quite sufficient to 

 v _ s produce a very marked difference in 

 the perspective of the successive layer.- 

 in the images. But with high powers 

 the difference in the two images does 

 not keep pace even when both eye- 

 pieces are ha,lf 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 

 mittedraysin observation with trans- 

 mitted light, it follows that under 

 these circumstances the difference of 



the 



turned through an angle of 90 to whole aperture -angle of the objec- 

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



of the incident and directly trans- 



mitted pencils, which only allow of relatively small differences 

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

 evident, however, that w T hen objectives 

 of short focus and correspondingly large 

 angle are used, a considerably greater 

 differentiation of the two images with re- 

 spect 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 illumination, though 

 it cannot be obtained by the simple 

 mirror, can be easily produced 1 >y using 

 with the condenser a diaphragm with two 



openings (fig. 82), placed in the diaphragm stage under the con- 

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

 of narrower or wider aperture and of greater or less inclination 



