115 , 



properly used. Generally speaking the objective may 

 be said to be spherically corrected when it gives the 

 best defined image ; that is, when the outline and 

 internal structure of an object of extreme thinness 

 appear in one plane. When, after the objective has 

 been focused on the outline of the object, it is neces- 

 sary to increase the distance to focus on the structure, 

 it is evidence that the objective is spherically over- 

 corrected and that the cover is too thick ; in adjust- 

 able objectives the correction collar must be brought 

 to its closing point, which means that the lenses are 

 brought in closer contact. When the objective must 

 be focused to a point beyond the outline of the object 

 to see its structure that is, brought closer to the 

 cover-glass it proves that this is too thin, and is then 

 said to be spherically under corrected ; to give the 

 proper adjustment in an adjustable objective in this 

 case the adjustment is opened the lenses are sepa- 

 rated. It requires a certain amount of study to dis- 

 tinguish these phenomena, and although it can be 

 done in well prepared specimens, I know of none 

 better than coarsely marked diatoms, such as P. 

 angulatum. 



Although I am aware that many eminent micro- 

 scopists do not favor adjustable objectives for every 

 day work, 1 must confess that I fail to see the force 

 of their arguments. From the foregoing it will be 

 seen that unless the cover-glasses are of a thickness 

 corresponding to that which was originally used, the 

 objective may be made to do imperfectly what is in its 



