436 MANIPULATION AND PRESERVATION OF THE MICROSCOPE 



dioptric beam ; 1 for diffraction spectrum of the first order ; 2 for 

 diffraction spectrum of the second order. 



When the back of an objective of '83 N.A. shows an arrange- 

 ment as in 



No. 1, then, although the structure will be invisible, 



it cannot be coarser than . . . 40,000 per inch. 



No. 2 80,000 



No. 3, then the structure does not differ greatly from 40,000 



No. 4 *0,000 



No. 5 .. 20,000 



No. r. 40,000 



It will be understood by the student that the preservation of the 

 in icroscopp, and its apparatus is a matter that must largely depend 

 upon his own action. The stand should be kept from dust, generally 

 wiped with a soft chamois leather after use, and when needful a 

 minute quantity of watchmaker's oil may be put to a joint working 

 stiffly. There is no better way to preserve this stand than to keep 



it either under a bell-glass or in a cabinet which is easily accessible. 



All objectives should be examined after use, and all oils or other 

 fluids carefully wiped away from them with old cambric which has 

 been thoroughly washed with soda, well rinsed and not ' ironed ' or 

 finished in any way, but simply dried. 



If chemical reagents are employed the cessation of their use 

 should become the moment for wiping with care the lenses employed ; 

 and all processes involving the use of the vapours of volatile acids, 

 or which develop sulphuretted hydrogen, chlorine, Arc., must never 

 lake place in a room in which a microscope of any value is placed. 



Dry elder-pith and Japanese paper are by some workers .sug- 

 gested for cleaning the front lenses of homogeneous objectives; but 

 while these are excellent, especially the former, we find nothing 

 Itetter than the simple cambric \ve suggest. 



Two or three good chamois leathers should be kept by the 

 worker for specific purposes and not interchanged. Cleanliness, 

 care, delicacy of touch, and a purpose to be accurate in all that he 

 Iocs or seeks to do are essentials of the successful microscopist. 



