1889.] MICROSCOPICAL JOURNAL. 55 



1.30, for 400 marks, or a little less than $100; and a y'^-inch, N. A. 

 1.40, for 500 marks, or almost $125 ; while Baiisch & Lomb Optical 

 Co. and H. R. Spencer ofler their homogeneous immersion objectives, 

 N. A. 1.40 and 1.38, respectively, at from 40 to So per cent, less, as 

 their catalogues will show. 



[Referring to the reports of his address which appeared last Sep- 

 tember, Dr. Detmers says, in contradiction, that he did not take mi- 

 croscopes, objectives, or accessories to Europe ; that he did not make a 

 test of skill with the Germans ; that he did not photograph objects in 

 competition with them ; and, in short, that no such fighting of objec- 

 tives as was described occurred.] 



Notes on the Substage Condenser, with Special Reference to that 

 of Professor Abhe. 



By the loiterer IN A MICROSCOPIST'S LABORATORY. 



This piece of illuminating apparatus is one of the most important and 

 valuable that the microscopist can possess. It may be used to increase 

 the illumination until the eye refuses to endure it, or the light bv its 

 means may be reduced to the faintest glimmer. With it the whole 

 aperture of the objective may be filled by a solid cone of light, and by 

 the use of the proper diaphragms or by moving the entire condenser 

 laterally, illumination of the greatest obliquity may be obtained for the 

 resolution of tests, or for the study of obscure structures of a certain 

 character ; and with the best of the modern substage condensers, black- 

 ground illumination of the most exquisite beauty may be accomplished. 

 The defining and resolving power of the objective are improved by its 

 use. Indeed, the best high power homogeneous immersions will not 

 do themselves entire justice without the use of the wide angled con- 

 densers now so common. 



In what is called black-ground illumination the object appears to be 

 self-luminous, gleaming with the vivid radiance of molten silver, seem- 

 ing to rest softly on a back ground of the blackest velvet. Living ani- 

 mals appear like moving creatures fashioned from moonbeams ; minute 

 particles shimmer and flash like silver stars ; a little heap of colored 

 sand grains seems a little heap of rubies and diamonds from Sinbatl the 

 Sailor's Valley of Gems. And to obtain such exquisite pictures it is 

 only necessary to obstruct the central beams of light by a circular 

 opaque disk, allowing the object to be illuminated by the light that 

 comes to it from the periphery. No rays reach the objective directly. 

 All must first enter the object and there be properly refracted or inflected, 

 or after passing through the object, they must be thrown back on it by 

 reflection trom the cover glass, so that under the beating of those waves 

 of light it shall appear to glow with a soft intensity indescribable. This 

 eflfect may be obtained, sometimes better and more easily, by substage 

 apparatus especially intended for the purpose, rather than by any sub- 

 stage condenser. 



The accessory just mentioned is a collection of two or three lenses 

 forming an instrument somewhat similar to an objective. It is fitted to 



