108 Hall, on a New Form of Condenser. 



similar amount of compression has been experienced by theni, 

 and a careful microscopic examination by both authors of this 

 paper shows that only a shght disturbance in the position of 

 some of the terminal cells has taken place. 



It would thus seem that the contraction of the gelatinous 

 silica into the solid hydrate differs materially from the condi- 

 tions that would result from a mechanical pressure acting from 

 wit])out,.as when water is squeezed out of a sponge, or from 

 a mere rush of molecules from the outer layers towards the 

 centre. 



On a New Form" 0/ Condenser ivith a Blue Tinted Field 

 Lens. By W. H. Hall, F.E.M.S. 



(Read May 13tli, 1868.) 



Some few months ago I was asked by several of the 

 members of the Cambridge Heath Microscopical Society to 

 recommend a condenser of such a price as to be consistent 

 with the sums paid for the cheap student's microscopes pur- 

 chased by them ; but not finding one suitable for this purpose, 

 I made some suggestions to Mr. Swift, of Kingsland Road, 

 who undertook to carry them out, and has succeeded so well 

 that I have thought it desirable to direct attention to the 

 result. 



There are two optical combinations, one — the cheaper — 

 sufficiently corrected for achromatism for ordinary purposes, 

 and connected with a suitable mounting; the other achro- 

 matic, and more elaborate in its mechanical arrangements. 

 Both forms are on the table, and will be understood by the 

 engravings attached to this paper. 



The under, which may be called the field glass, is a plano- 

 convex lens of low curvature, made, if intended for use with 

 artificial light, of blue glass of sufficient depth of tint to 

 neutralize the yellow rays, and produce a soft daylight eifect, 

 ■which I have found very grateful to the eyes in long-con- 

 tinued observations. A similar shaped lens of colourless glass 

 is provided for solar light ; this condenses the light on a deep 

 plano-convex combination of plate and flint glass, having 

 somewhat different curves in the cheaper and more expensive 

 forms, and worked at a much less cost in the one than in the 

 other. The angle of light given by each is, however, the 

 same — about 110°. 



