166 



TESTING THE MICBOSCOPE. 



We thus arrive finally at the result, that the defects of correction, 

 which arc rendered apparent ly the given method of testing (in which 

 only the half of the objective is optically effective), arc to 'be referred 

 solely to the objective in all cases where they are readily seen. 



The second point we have to consider, namely, the chromatic 

 aberration of the eye, may in principle be regarded as solved. 

 That the eye is affected with a very marked chromatic aberration, 

 may be easily shown by covering up half of the pupil and directing 

 the gaze to any bright object. One margin will then appear 

 distinctly blue, just as in the microscopic image, and the other 

 orange or yellow. The only question remaining is whether this 

 chromatic aberration can increase or compensate that of the 

 Microscope to any considerable degree. To decide this point, it is 

 only necessary to cause the pencils of light proceeding from the 



It 



eye-piece to reach the retina through a part of the pupil as 

 excentric as possible. The eye should be placed at about the 

 height where these pencils of light cut the optic axis (at the so- 

 called eye-point), and moved so far to the right and left that the 

 microscopic image is only just visible. It will then be observed 

 that the red and blue margins appear fainter or more intense, 

 according to the position of the eye, and that the colours are often 

 reversed and even disappear wholly. These phenomena may be 

 explained by taking the action of the naked eye upon white light 

 as the starting-point. If E E (Fig. 96 and 97) is the principal 

 plane of the eye, E the retina, and P the pupil, which is half 

 covered in the manner indicated by the diaphragm B ; a colourless 



