43 



difference is considerable. In both objectives and 

 eye-pieces the lenses decrease in size with the increase 

 in power and consequently give less light ; and while 

 this one objection exists in the objective an addi- 

 tional one occurs in the eye-piece, in that the eye 

 must be brought closer to the eye-lens and must be 

 kept more strictly in the optical axis, which at a long 

 sitting becomes fatiguing. 



Between the 1 inch and 2 inch the choice should be 

 determined by requirements and individual prefer- 

 ence. All responsible manufacturers and dealers make 

 up such outfits of stands, objectives and eye-pieces, 

 which experience has taught them are most generally 

 useful. 



It is a safe rule to follow in all work on recognized 

 forms ( objects of which the structure is known ) not 

 to use a higher power than is necessary to properly 

 study them. 



Eye-Piece Huyghenian. This is now in gen- 

 eral use, and consists of two plano-convex lenses. It 

 ^~ -\ r ~^> receives its name from the 

 v/ \J inventor, who first applied it to 

 the telescope. The eye-lens is the 

 small lens nearest the eye, and 

 the fie 'Id- lens, or collective as it is 

 also called, is the large one near- 

 est the objective. A diaphragm 

 is placed between them, and 

 gives a sharply defined field. 

 This eye-piece is also called neg- 

 ative, as its focal point is between 



