ORDINARY CONDENSER IMPROVED, &C. 317 



tion the rays in their passage through it will be unrefracted, and, 

 consequently, not in the least detrimental to the definition. 

 Possibly, by cementing it down with Canada balsam to the plane 

 surface of the condensing lens, it might be equally advantageous ; 

 but this remains to be tried. Its next best place will be imme- 

 diately above or below the diaphragm. 



Now, when so much nicety in the perfection of the illuminating 

 pencil is essential to its well-working, it is most surprising that 

 such a thoroughly unscientific arrangement as the introduction of 

 ground glass, which breaks up the integrity of the rays, and scatters 

 them in every direction, should, in the present day, be tolerated even 

 for a single moment ; but it is to be hoped we have seen nearly the 

 last of it. Let any such object as the Heliopelta Metii, for instance, 

 be closely examined under the tinted glass properly adjusted, and 

 then let it be quickly transferred to the ground glass in the place of 

 the other, and much of the detail will be found to be lost or greatly 

 obscured, and as the present arrangement admits of almost universal 

 application, and may be adopted at very little cost, and giving such 

 very superior effects, I have no hesitation in pleading in its behalf 

 for an unbiassed trial. There has been some difficulty in obtaining 

 the exact tint, and which has delayed the completion of these 

 remarks ; but having at last succeeded, I have forwarded a specimen 

 for comparison, as it is important that neither too much green nor 

 too much blue and no violet should be present ; while as to the 

 thickness of the glass, .it seems preferable to employ a thick glass 

 lightly tinted instead of a thinner substance more deeply shaded. 

 With one thickness of the present sample, a most agreeable effect 

 will be obtained in the daytime, by taking bright light from opposite 

 to the sun ; but with a gaslight or mineral-oil lamp, it will require 

 two thicknesses to sufficiently neutralize the dazzling brilliancy, 

 whether the lisrht be taken direct or reflected from the mirror. 

 "Where, however, two thicknesses are required, these should be ce- 

 mented together with Canada balsam, and also a piece of flat 

 covering glass should be cemented upon each outer side, as affording 

 a better surface, and one that will not easily be dulled by friction in 

 cleaning. 



