A COMPABISON OF CONDENSERS 



315 



We give below a list presenting the most important features of 

 the most important condensers, which we believe will be of service 

 to the student and worker. 



The aplanatic aperture given in the table means the N.A. of the 

 greatest solid cone a condenser is capable of transmitting, the 

 source of light being the edge of the flame placed in the axis. 



The cone transmitted by any condenser is assumed, for practical 

 purposes, to be a solid one. so long as the image seen at the back of 

 the object-glass when the eye-piece is removed (the condenser and 

 flame being centred to the optic axis of the objective, and the source 

 of light focussed by the condenser on the object) presents an un- 

 broken disc of light. 



The moment, however, the disc breaks, that is, black spots 

 appear in it, or its periphery breaks away from its centre, then, as 

 we have shown above, spherical .aberration comes into play, and the 

 limit of aperture for which that condenser is aplanatic has been ex- 

 ceeded. 



The values of the first sixteen and of Nos. 22, 23, and 25 have 

 been obtained from actual measurements ; the others are from the 

 estimates of the makers. 



The limit given in the table is for the edge of the flame as a 



