In all objects other than diatoms it is generally 

 difficult to form an opinion as to spherical correction. 

 If from preconceived idea of what an object should 

 show it fails to meet expectations or is hazy where one 

 expects it to be distinct, and being certain that the 

 objective and eyepiece are properly cleaned, it may 

 generally be ascribed to lack of proper correction. By 

 focusing either above or below the proper focal plane 

 there will be an enlargement of the outlines of the 

 object or a coma which gradually enlarges as the objec- 

 tive recedes from the proper focal point. 



If the expansion is greatest when the objective is 

 elevated, there is spherical over-correction and the tube 

 length should be decreased. 



Should the expansion be greatest when the objective 

 is lowered, there is spherical under-correction and the 

 tube length should be increased. 



Chromatic Aberration. This may be judged 

 as described under " Chromatic Aberration" in a 

 previous chapter, page 85. 



Cover Glass. We have thus far not considered 

 the cover glass, except to show its influence on the 

 optical performance of objectives. In preliminary 

 examinations of solid objects with low powers it may 

 be dispensed with, but where fluids are used, whether 

 with low, medium or high powers, it should always be 



140 



