The cover glass may truly be called a necessary 

 evil ; for, while absolutely required in microscopical 

 investigations, there is no adjunct to the microscope 

 that has been and is productive of so much evil, and 

 has done so much to retard the utilization of benefits 

 made possible by the advance in the construction of 

 objectives. 



It must be remembered that the majority of objec- 

 tives will always be dry, and it is an unfortunate 

 circumstance that with this class of objectives the 

 influence of variation in thickness of cover glasses is 

 most apparent; but- since it is so, all possible aid 

 should be given to enable the student to obtain results 

 which closely equal those obtained with the conditions 

 under which the objectives were originally corrected. 



With oil immersion objectives a variation in thick- 

 ness is not very appreciable, provided, however, that 

 the fluid is of the proper consistency, as there is pract- 

 ically no refraction between the cover glass, immersion 

 fluid and front lens of objective. If the fluid, however, 

 does not have the same refraction as the cover the 

 sperical aberration may be as pronounced as with dry 

 objectives, as it becomes more noticeable on account of 

 higher power. It is therefore important to purchase 

 the fluid from the maker of the objective or under a 

 guarantee of the index of refraction. 



Objectives with fixed mountings, such as are ordi- 

 narily used, in which the lenses have fixed relations 



92 



