51 



r 



with the addition possibly of a very small bottle of gold-size for 

 fixing the cover-glass, in tightly closed bottle, which will remain 

 :good and fluid for a number of years, for those livin^ at a 

 "distance from town and out of reach of shops. C. F. Kousselet.] 



AN IMPROVED CHROMATIC CONDENSER. 



By E. M. Nelson, F.R.M.S. 

 {Read March 26th, 1912.) 



Every one who has used an Abbe condenser (invented 1874) must 

 l3e aware of its spherical aberration, for its aplanatic aperture 

 is so small that it will only fill the back lens of an objective that 

 has a very low N.A. The difference in the focus of its central 

 rays and those used for dark-ground illumination is so great that 

 they cannot be both used at the same time. In fact, the spherical 

 aberration in its outer zones is suflBciently large to hinder its 

 fiction as a dark-ground illuminator, for if rays about N.A. 1*0 

 are in focus those about N.A. 1-2 are out of focus. 



The Abbe condenser owes its popularity first to its cheapness, 

 and secondly to the advertisement it receives from those who, 

 not knowing how to use the microscope and its lenses properly, 

 invariably employ their highest powers in a degraded manner to 

 examine low-power objects. 



It would seem that with a little trouble something might be 

 done to improve it, for it is obvious that one fault about this 

 condenser is that it attempts too much : if it were cut in two, 

 so to speak, it would perform better. In brief, would it not be 

 advisable to have a simple form of n on- achromatic dark-ground 

 illuminator that would be capable of doing real serviceable work, 

 and also for ordinary work a cheap narrow-angled chromatic 

 condenser with its spherical aberration at a minimum ? 



The first part has been done, and the dark-ground illuminator 

 was exhibited to you a little while ago. Those who have used 

 it say that it gives very satisfactory results. It is a three- 



