252 Dr. Goring on Chevalier's 



apertures of their object-glasses to the requisite angle, and have 

 moi'eo\er arrived at the true method of adjusting them, so that 

 they are now free from those objections which appUed to Mr. 

 Lister's, and are in all respects unexceptionably finished. 



I know not if any dispute will ever arise hereafter, as to who 

 is to be considered as the original maker of effective aplanatic 

 object-glasses for microscopes *. It is of very little consequence 

 in the present instance, for it so happens that Mr. Tulley and 

 Messrs. Chevalier have been so totally unconnected with each 

 other, and have worked upon such totally different principles, 

 that it must be evident, on the most superficial consideration, 

 that both are entitled to the honour ; nevertheless I apprehend 

 it can be proved that Mr. Tulley made an effective one before 

 'the Chevaliers, having completed his in March 1824. The 

 date affixed by Chevalier to his first instrument is 1825t. 



It requires moreover a stretch of complaisance, not to be ex- 

 pected on this side the Channel, to be enabled to admit that the 

 best double object-glass is {taken singly) effective ; or that, in 

 consequence, Chevalier made an effective one until he had en- 

 larged his apertures, and combined two together']^, which combi* 

 nation is not to be met with in his primitive instruments. Mr. 

 Lister, (to whom the public is mainly indebted for the pre- 

 sent eclaircissement concerning Chevalier''s instrument,) has, 

 by a peculiar method of his own discovery, measured the 



* The question must naturally resolve itself into this point, for achro- 

 tnatics for microscopes, (as they are called,) were made by Dollond, 

 Martin, and Pollard, many years ago ; the only objection to them was, 

 that they were not effective, consequently, nominal only, and useless. I 

 defy any man to produce an effective object-glass, which can satisfac- 

 torily be proved to have been made before 1824. 



+ See ihefirst instruction published by Messrs. C. along with their 

 microscopes : " Microscope Achromatique selon Euler," &c. I appre- 

 hend there is a misdate in Messrs. C.'s letter at the end of this paper, 

 where they state they have made the achromatics since 1824, with four 

 objectives, which implies that they also made them before that period, 

 but in another way. 



X Mr. C. Tulley recommended the combination of two achromatics 

 together from nearly the beginning of his son W.'s labours upon them, 

 who rejected the idea, together with myself, as giving rise to too great a 

 complication, always supposing that triple glasses must be used. I 

 myself combined two triple achromatics together, for experiment salve, in 

 a very early stage of our proceedings, but liked not the result, though 

 effects were certainly produced by the composition, which could not be 

 obtained from the best individual triple one. 



