514 



lens dark-ground condenser of minimum aberration ; it is a 

 possible and inexpensive construction, because it is not hampered 

 with a central part. Now Mr. Curties will show you the central 

 part unhampered with an oil-immersion margin. This is also 

 a non-achromatic triplet of 0"65 N.A., and of minimum aber- 

 ration. 



A chromatic condenser of wide angle invented by the Rev. W. 

 Kingsley in 1850 was made by E,oss (figured in Quekett, " On 

 the Microscope," 2nd ed., p. 500, 1852). This was not in- 

 tended to supplant the superior achromatic condenser, but was- 

 designed to be used excentrically for obtaining blue light for 

 resolving striae on diatoms. The blue light, formed by the 

 chromatic dispersion of the lens, resembled that from an 

 achromatic condenser illuminated by spectrum blue light. By 

 this means the pearly light, formerly so much sought after by 

 striae resolvers, was obtained. 



The formula upon which this condenser is constructed, two 

 menisci and a bi-convex, differs from that of Kingsley's, which 

 has two bi-convex lenses and a meniscus. Its aperture, instead 

 of being made as large as possible, is kept to 83, so that it may 

 be used with three quarter cone ordinary dry objectives ; its 

 aberrations are at a minimum, and it is made of low-dispersive 

 Jena glass. These two illuminators therefore together form 

 an improved Abbe condenser. 



AN APLANATIC SPOT LENS. 



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



{Read March 2C,th, 1912.) 



Mr. Stephenson invented, in 1879, an immersion dark-ground 

 illuminator, which consisted of a plano-convex lens mirror ; it 

 is figured in Journ. R.M.S. vol. 2 (1879), p. 36. In the de- 

 scription it is stated, first, that "if used with a dry lens of the 

 highest power on a balsam-mounted object the light, unable to 

 pass the upper surface of the covering-glass, is thrown back on 



