The Microscope. 5 



Considerable also depends upon the perfection of the eye- 

 piece. I believe that as a rule too little care is devoted to it ; 

 at any rate, it is certain that while any Hivyghenian eye-piece 

 for a telescope can be used on a microscope, very few which 

 have been made for this can be used on a telescope, and while 

 it is true that no such perfection may be required in the former 

 it leaves such an indefinite range that it may become difficult 

 to place a limit for the perfect and imperfect. In all work, and 

 especially in testing, it should be seen that the eye-lens as well 

 as held- lens are perfectly clean. 



Among the absolutely necessary conditions in judging of 

 the quality of an objective are perfect specimens, especially if 

 they are sections. A thick object obstructs the light, and gen- 

 erally makes it necessary to go through so many layers or 

 planes that it is difficult to get any one distinct; the impression 

 may thus easily be given that the objective is at fault, The 

 difference between two objects of the same nature may be so 

 great, that while with one the objective may be condemned as 

 imperfect, it may with the other appear of extraordinary excel- 

 lence. 



In conclusion, I will say that there may be other conditions 

 which may influence the performance of a lens, and to acquire 

 the power of eliminating them requires considerable experience. 

 When an objective does not correspond with the claims of the 

 optician, judgment should not be passed upon it until after 

 repeated trials have been made, in all of which the above points 

 should not be lost sight of. 



THE WORKING SESSION. 



ALLEHT M'CALLA. 



THE Executive Committee of the American Society of Mi- 

 croscopists have very wisely appointed Mr. C. M. Vorce, of 

 Cleveland, Ohio, as committee in charge of the session for prac- 

 tical work at the next meeting of the Society, and it is to be 

 hoped every one who can help will heartily co-operate in 

 making that session a grand success. As the original proposer 

 of the scheme I feel greatly interested in its growth. It was, 

 from all accounts, a most interesting feature of the Rochester 

 meeting, which I was unable to attend, and Mr. Griffith is en- 



