12 AQUATIC MICROSCOPY FOR BEGINNERS. 



different make from those accompanying his instru- 

 ment,, he was forced to buy a little piece of apparatus 

 called an adapter, one end of which was made to 

 screw into the microscope body, the other to receive 

 the objective. At the suggestion, however, of the 

 Royal Microscopical Society of London, all objectives 

 and stands now have screws of the size recommended 

 by that society, and therefore called the "society- 

 screw." Only the very cheapest stands of the present 

 day, or those having the least value as instruments 

 for serious investigation, are without this screw, 

 and they are usually supplied with what are termed 

 French triplets. These are miserable lenses that 

 should always be shunned, as they will do the observer 

 more injury than much time can remedy. 



It is true that before the optician, especially before 

 the American optician, began to make really good ob- 

 jectives at moderate cost, these French triplets were 

 extensively used, and are said to have done some good 

 work. But at what expense ? Not at the expense of 

 any great amount of knowledge or skill displayed in 

 their manufacture, for the lenses were ground, 

 mounted singly, and then combined in an experimental 

 way, two or three, it is said, being selected at random 

 from a basketful, screwed together, and examined on a 

 microscope. ' If the result was considered satisfac- 

 tory, all was well; if not, one or more of the lenses was 

 replaced by others also selected at random, and the ex- 

 periments were continued until the objective was con- 

 sidered passable and salable. Such, at least, is the 

 credible story. Their expense, therefore, was not in 

 the making; it was in the irn,perfect image, in the 

 great loss of light, in the injury of the eye due to the 



