THE ROUTINE MICROSCOl'E 61 



find it sufficient. But it is only about half the capability 

 of the objective. In order for this objective to perform 

 its utmost, without glare or dullness, it needs a solid 

 condenser cone of about 1.2 aperture, given by a correctly 

 adjusted achromatic immersed condenser, or by an apla- 

 natic chromatic condenser with yellow-green light screens. 

 The tube length should be standard, and the source of 

 light equal to (or less than) the source-field. The cover- 

 glass must be 0.17 millimeter thick, or the tube length must 

 be altered to partially correct for a thinner cover-glass 

 (thicker ones should not be used). Then the objective 

 should give a magnification of 1,250, the image of a well- 

 stained object being about as sharp and bright as that 

 given by the corresponding 10 objective. 



The dry objective of about 40 initial magnification and 

 0.85 aperture, with a 5-times eyepiece, gives 200 total 

 magnification; instead of the maximum useful magnifica- 

 tion of over 800, with full light and sharp definition, given 

 when all adjustments are correct. This reduced magni- 

 fication (200 times), of the 40 objective, requires only a 

 minimum condenser cone; which could even be supplied 

 by a concave mirror, all adjustments being correct. But 

 a nearly faultless magnification of 800 is only to be attained 

 (cover-glasses being slightly under 0.17 millimeter thick, 

 and other optical errors having been avoided) by a solid 

 corrected condenser cone of 0.8, with yellow-green light. 



There seems no reason, except custom, why a routine 

 microscopist should necessarily use an instrument capable 

 of more magnification than he requires. Objectives of 

 lower aperture are easier to use, and cost less. With the 

 difference in cost, an eyepiece with a higher eyepoint could 

 perhaps be obtained. The required magnification could 

 sometimes be got as well with a 20 objective, as with a 40 

 objective (as Beck and others have advised) ; the 20 objec- 

 tive not being so sensitive to shght differences of cover- 

 glass thickness as the 40. The achromatic 20 objective 

 has an aperture of 0.4 or 0.5, and can be used with a 10 

 or 15 eyepiece of high eyepoint, giving a magnification 



