14 AQUATIC MICROSCOPY FOR BEGINNERS. 



before the glass comes to the grinding-tool. Objec- 

 tives are usually a combination of several lenses, but 

 the union is not accidentally perfected. The maker's 

 knowledge of abstruse optical laws tells him the pre- 

 cise result to expect from the combination of lenses of 

 certain forms made from glass of a certain chemical 

 composition. He is the master ; his objective is a 

 masterpiece. The owner of a good objective must 

 not treat it carelessly. He should treasure it, for it is 

 not a common thing. When not on the stand in use, 

 it should be kept in the brass box supplied for that 

 purpose, and it should never be left on the stand 

 when not in actual employment. 



The part of the brass mounting of the objective 

 which bears the screw is the back; the opposite end 

 which shows a small flat surface of glass is the front, 

 or, as it is often styled, the front lens. The glass of 

 this part is soft and easily scratched; therefore, take 

 care not to let it touch anything hard; especially 

 avoid any gritty substance, or a dirty rag that may 

 hold a minute particle of sand or of hard dust. And 

 never touch it with the fingers, as the oily exudation 

 from the skin will soil it and interfere with the clear- 

 ness and the beauty of the image. I the front lens 

 becomes accidentally stained, or is soiled by long use, - 

 the objective should be sent to its maker, who, can 

 clean it, without the great risk that its owner would ex- 

 pose it to if an attempt should be made, to wipe the 

 glass. If fine dust adheres too :closely to be dislodged 

 by the breath, ravel out the edge of a piece , of very 

 clean old Jirjen or musJin, ,and with the fringe thus obr 

 tained gently sweep the isurfaceij ; ,, , . 



If the front lens piust be wiped, by the microscopist/.. 



