130 HOW TO SEE WITH THE MICROSCOPE. 



collar be adjusted by the maker to rotate under high 

 pressure only, regard this with suspicion, and examine 

 for back-lash closely, or what is perhaps the better 

 ^vay, get the opinion of a skillful mechanic. 



The models of some of our American objectives are, 

 111 the opinion of the writer, far too large and clumsy, 

 .and the fronts too large and too flat. We decidedly 

 prefer the conical front the more conical the better. 

 The new tenth of Mr. Herbert Spencer, which we have 

 before found occasion to mention, was, beyond all cavil, 

 the most beautifully mounted glass we had ever seen, 

 and in which the last-named objections were almost 

 wholly avoided. 



NOMENCLATURE OF OBJECTIVES. 



American and English microscopists usually class 

 their objectives on the basis of their focal length, it 

 being arbitrarily assumed that the inch s^lass worked 

 with ten-inch tube and with two-inch eye-piece, should 

 give an amplification of fifty diameters. Hence, the 

 half-inch glass would give one hundred diameters, the 

 one-fourth two hundred, the one-eighth four hundred, 

 the one-tenth five hundred, and so on. It very seldom 

 happens, however, that an object-glass will exactly re- 

 spond to the designation given it by the maker some 

 opticians over, while others under-rate their instru- 

 ments ; and then again with an adjustable glass the power 

 will change in different positions of the adjusting collar, 

 the amplification being greater with the systems at 

 closed than when at " open-point." Even when due 



