100 



The writer may be pardoned for introducing at 

 this point extracts from a paper which he read a 

 number of years ago. 



" The cover glass may truly be called a necessary 

 evil ; for, while absolutely required in microscope 

 investigations, there is no adjunct to the micro- 

 scope that has been and is productive of so much 

 evil, and has retarded the utilization of benefits 

 made possible by the advance in the construction 

 of objectives, so much as it. 



" It must be remembered that the majority of 

 objectives will always be dry, and especially so 

 when improvements, which we hope are still to 

 be made, are accomplished. 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, we should, if 

 possible, provide an agency which, eliminating the 

 personal factor of efficiency, will give, under all 

 conditions, results closely equal to those under 

 which the objectives were originally corrected. 



" It is surprising to see how little attention is 

 paid to this subject in the large majority of 

 the standard works on the microscope. Almost 

 all books give carefully prepared illustrations and 

 descriptions showing the effect on the course of 

 light of the interposition of the cover glass, and 

 after giving conclusive evidence of its disturbing 



