SUBSTAGE CONDENSERS, ETC. 101 



and immersion lenses, viz., to make one homogeneous system so as to lose no rays, at 

 least, not more than possible : hence immersion condensers give much more light. 



We have only tested two high-angled immersion condensers : one, having a N. A. 

 of about 1-35 by Conrady, is of exceptional merit ; its aplanatic cone being almost, if 

 we are afraid to say absolutely, as large as the N. A. ; and the other by the old 

 established firm of Powell and Lealand, which is close upon N. A. 1-40. This has 

 a peculiar mount which admits of the use of several forms of diaphragms. It is said 

 to be by Dr. Van Heurck of exceptional merit, and that we are not surprised to hear, 

 for we have never heard of anything of an indifferent nature being sold by the firm 

 in question. It is shown in Fig. 53.^ 



Somewhat recently Beck, of Cornhill, introduced a high angle condenser. We 



Fig- 55 



have no personal knowledge of this piece of apparatus, but understand from those 

 qualified to judge that it meets with all the requirements of the microscopist. It 

 is shown in Fig. 54 with an iris diaphragm attached. It is said to have a N. A. 1-3. 



Lastly, in using all condensers, the distance of the lamp from the mirror, plus the 

 distance of the mirror from the substage condenser, is important, and it would be well 

 if makers marked this conspicuously on the mount. With some opticians it is greater 

 than others, and can mostly only be found by direct experiment. Mr. Conrady has 

 fixed a total distance of six inches, which we regard as a subject of great regret, as it 

 seems too small. 



All condensers should be mounted so that they can be centred with the optical 

 axis of the microscope to enable them to perform at their best. This is very important, 

 especially with those which are badly corrected for spherical aberration, thus forming 



* It should be mentioned that this condenser is not apochromatic as Dr. Van Heurck accidentally reports it to be • 

 the firm distinctly deny this and much wish it to be known, as it occasionally leads to misunderstanding. 



