64 



to this thickness and give the best results with the 

 thinnest of these. The same thickness is also used 

 on test objects, but they are generally not of as much 

 uniformity as might be desired. Objectives some- 

 times have such an extremely short working-distance, 

 that it is necessary to use the thinnest of No. 1, but 

 as these are usually provided with adjustment for 

 correction, their injurious influence is not so much 

 felt. The thickest covers are most comfortable to 

 handle and may be used with low power objectives 

 without much sacrifice of definition. 



The writer takes the liberty of inserting in this con- 

 nection extracts from a paper which he recently read 

 before the American Society Microscopists and 

 which he hopes will give further information on this 

 subject. 



" The cover-glass may truly be called a necesssary 

 evil ; for, while absolutely required in microscopic 

 investigations, there is no adjunct to the microscope 

 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 objec- 

 tives will always be dry, and especially so when such 

 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 var- 

 iation in thickness of cover-glasses is most apparent ; 

 but since it is so, we should, if possible, provide an 



