so made that the objectives are parfocal: i. e., when one 

 lens is in focus the others on the nosepiece will be in 

 fairly good focus when they are swung into the optical 

 axis. They are also approximately centered so that a 

 poinf in the center of the field of one lens will be in the 

 field of the others. To accomplish this, each set of 

 objectives must be especially fitted to their particular 

 nosepiece. Care should then be taken in the laboratory 

 not to interchange objectives. Be careful not to bend 

 the nosepiece in any way so that the objectives will be 

 thrown out of center. Unless you are positive that your 

 lenses are parfocal, always focus up slightly before turn- 

 ing from a lower to a higher power; otherwise you are 

 apt to swing the front of your objective against the 

 cover glass and injure both the specimen and objective. 

 Remember that objectives made parfocal for one tube 

 length arc not parfocal for a different length. If occasion 

 occurs for screwing the objectives on or off from the 

 nosepiece, always use both hands, never letting go of the 

 objective entirely, so as to preclude any liability of 

 injuring them by dropping. 



THE OPTICAL PARTS 



With the stand cleanliness is a virtue; with che ob- 

 jectives it is also an absolute necessity. If the lenses 

 are dirty they should be wiped gently with Japanese 

 lens paper, which can be obtained from any dealer in 

 microscopical supplies. It is so cheap that one can 

 hardly afford to use anything else on his lenses, espe- 

 cially the objectives. If the lens paper is not obtainable 

 a soft old linen handkerchief is best, providing it is clean. 

 Avoid chamois skin. The natural oils in it soil the sur- 

 face of the lens, and its aptitude to catch and hold dirt 

 makes it unsafe. 



Never rub a lens hard with anything. Avoid touching 

 11 



