96 



Under-correction will show when the mirror is 

 swung to the left and when the object is fringed 

 with orange on the left and the blue-violet on the 

 right. Or, over-corrected when the colors are 

 reversed. 



Another method is that recommended by 

 Naegeli & Schwendener in their work on the 

 microscope. The right half at the front or back 

 of the objective is covered by black paper or tin- 

 foil so that only the other or left half remains 

 optically effective ; take for an object a line or dot 

 of light which can be easily produced by blacken- 

 ing one surface of a slide with chimney soot and 

 drawing a line across it with a point. 



Under-correction shows when the image has on 

 the right side a violet or blue border and on the 

 left a red or orange colored border. 



Over- correction shows on the other hand when 

 the left side appears violet and the right red or 

 orange. 



Spherical Aberration and Cover Glass. As 



we have stated there is a residue of chromatic and 

 spherical aberration in all ordinary achromatic 

 lenses and objectives. The use of cover glass 

 influences the spherical correction and while not 

 appreciable to any extent in low powers, it is very 

 sensible in the medium and high powers. If we 

 refer to Fig. 33 it will be remembered that the 

 rays from the object do not uninterruptedly reach 



