250 Dr. Goring on Chevaliers 



This object-glass is under corrected in point of colour, and 

 wants to be made longer in the focus to be achromatic. The 

 excess of uncorrected spherical aberration is in the convex lens ; 

 the glasses are not well ground, centered, or adjusted ; the same 

 appearance of tarnish as in 14 ; bears its clear aperture for the 

 middle of the field on most transparent objects, but must be 

 cut off to 0.2 for opaque ones. 



Both of these object-glasses are ineffective upon test objects, 

 from want of sufficient power and aperture. 



4.) Focus about 0.43, clear aperture 0.23, original stop 

 0.09, perfectly achromatic. The uncorrected spherical aber- 

 ration is in the concave ; centering and grinding very fine, but 

 in very bad adjustment ; shows some transparent test-objects 

 pretty well with its clear aperture, and, cut off to about 0.16, 

 performs Avell on many opaque oties. 



2.) Focus about 4.7, clear aperture 0.21, no stop, perfectly 

 achromatic, surplus of spherical aberration in the concave as 

 before ; centering and grinding very fine ; adjustment tolerable j 

 in other respects very similar to 4. This object-glass being 

 adjusted, does more singly on test-objects than any other, and 

 carries an aperture of 0.16 well on opaque bodies, showing the 

 lines on the diamond-beetle's scales strong and well cut out. 



Coji^bination of 4 find 2 — (quadruple.) 



I am happy to be able to speak in terms of almost unquali- 

 fied approbation of this composition. It, of course, surpasses 

 the performance of any single triple-glass, on those test-objects 

 which require extravagant angles of aperture. The field also 

 is good all over, or at least would be, if the glasses were in 

 adjustment, which is the only drawback upon it. The focus 

 of the combination is only 0.26, yet it performs admirably on 

 transparent test-objects with its naked aperture of 0.23, and is 

 very fine on opaque ones with 0.16, and doubtless would carry 

 0.2, if the adjustment was duly carried into effect. 



Messrs. C. have, I think, most assuredly here hit upon one. 

 of the very best compositions for the object-glass of a micro- 

 scope; all the imperfections of double object-glasses, taken 

 singly, are here done away, while their thinness and agglutina- 

 tion into one mass allows of their combining together almost 

 as if they were simple plano-convex lenses, leaving moreover 

 abundance of space for the illumination of opaque bodies. 



