254 Dr. Goring on Chevalier's 



with a large angle of aperture. If a man aims at perfection, 

 and wishes to distinguish himself in this branch of optics, let 

 it be done by working perfect triple glasses of 0.2 and 0.3 inch 

 focus, with 0.1 and 0.15 of perfect aperture, like those of 

 Mr, W. Tulley and Mr. Dollond, or deeper still, if he is able ; 

 and it is with the most cordial satisfaction that I am enabled 

 to inform my readers, that Messrs. Chevalier (duly appre- 

 ciating the regular triple construction as the true form for 

 the microscope) have applied themselves diligently to the 

 manufacture of this species of objective, from which they have 

 already had excellent results. It is a fundamental principle, that 

 all superfluous refractions and reflections are to be avoided in 

 the construction of optical instruments. As a radical reformer of 

 microscopes, I can tolerate no abuses in them, or show any 

 quarter to their abettors. Messrs. Chevalier have also under- 

 taken the manufacture of achromatic and catadioptric micro- 

 scopes, after the fashion of those made by Professor Amici, of 

 Modena, which were so much and so justly admired by the 

 cognoscenti of this country. 



It only remains for me to observe, that though two double- 

 cemented object-glasses form the most perfect combination 

 from the fewness of their surfaces, and consequent brightness 

 of their image, yet a fusion between the Tulleian and Eulerian 

 constructions seems to be the most convenient for general use ; 

 by this, of course, I mean a triple glass with a double one, 

 to apply before it occasionally, a la Lister. Mr. Dollond and 

 Messrs. C. have demonstrated that two object-glasses may be 

 combined with the best effect, which are both good, and work 

 well separately ; but Mr. Tulley has constructed a double 

 one, which, useless by itself, when applied over a triple one, 

 (made to act singly,) corrects that excess of spherical aberra- 

 tion in the concave lens, (by its own excess in the convex,) 

 which, when the aperture is large, is the eternal vice of all the 

 best single and compound object-glasses for diverging rays 

 which I ever saw. This is perhaps the ultimatum of improve- 

 ment, though a quadruple one, on the same plan, might have 

 the advantage in greater light and clearness, from its simpli- 

 city, and the paucity of its surfaces. 



The quadruple or quintuple object-glasses are those which 



