35 



dicial to an objective ; for, if in two, one shows the 

 structure of an object with a slight amount of it, the 

 other does not show the structure but gives a nearly 

 colorless image, it goes without argument to say that 

 the first gives the best results and is therefore pre- 

 ferable. 



Within recent years new kinds of glass have been 

 found by means of which the amount of remaining 

 color has been reduced, and objectives constructed 

 with it are called apochromatic. 



If on increasing the distance between the objec- 

 tive and object the latter shows a marked blue color, 

 and when the distance is decreased a yellow-red 

 color, the objective is chromatically under-corrected j 

 if, however, the conditions are reversed, if the object 

 shows a yellow color when the distance between it 

 and the objective is increased, and blue when 

 decreased, it is over-corrected. 



Resolving Power. This is the quality in an 

 objective by which we are enabled to see the intricate 

 structure and finer details in an object. It depends 

 upon the amount of angular aperture, the correction 

 of the chromatic and spherical aberrations, and of 

 course upon the perfection of the mechanical work. 

 The power of resolving in an objective is indicative 

 of the perfection of the microscope, for it is almost 

 entirely dependent upon it for its quality. 



When an objective is said to resolve a structure or 

 a certain number of lines, it means that it shows 

 them under certain conditions of light. It may 



