208 



Transactions of the Society. 



and no more. Again, witliout venturing to say there is anytliing 

 in the suggestion, may it not be that in the search for Bacteria a 

 dark-ground ilhimination may be of use. Every microscopist knows 

 that with a black ground, even the most minute speck ghstens in 

 the hght, and how a single delicate diatom in balsam, which it might 

 take one an hour to find in a bright field, instantly becomes visible 

 on the dark ground. 



The illuminator (fig. 44) consists of a segment C D E (cut 

 from the edge inwards) of a plano-convex lens of crown glass of 



Fig. 41. 



Eadiiis of curvature = 1 inch. 

 AtoB = 0-56 

 AtoC = 0-34 

 C to D = 0-20 



D to E silvered curved surface 

 F segment of pkiuo-convex lens. 



1 in. radius of curvature, the lens having a diameter of 1 • 2 in,, 

 and therefore a thickness of • 2 in., the upper surface being black. 

 The segment has a length of ■ 56 in. and a depth of • 5 in., and 

 is therefore almost a square. The curved surface is silvered as 

 in the catoptric lens described by me in 1879.* It is cemented 

 to a rectangular piece of flint glass A B C E, the refractive index 

 of which is 1'652. The thickness of the flint being 0'34 in., 

 makes the total thickness, when the two are cemented together, 

 rather more than half an inch. 



The object of the flint glass is twofold. In the first place it 



* See this Journal, ii. (1879) p. 36. Several catoptric lenses have been made 

 for me of light, medium, and dense flints since 1879 ; one of these, of the numerical 

 aperture of 1'561, was used by me to illustrate my paper on the visibility of 

 minute objects in phosphorus, in 1880 ; but one wade for me since then by Mr. 

 Zeiss, of dense flint, has a numerical aperture of 1*628, in the same material, on 

 a slide 0-03 iu. thick. 



