Di\ Goring o>i Achromatic Microscopes. 411 



either glass singly. There is, however, an inevitable dull- 

 ness produced in the image of an opaque body, by the mul- 

 tiplicity of the surfaces of the six lenses, somewhat similar to 

 what occurs when an ordinary object-glass is dimmed by 

 moisture ; with transparent objects, this defect is scarcely 

 felt. I need not say that an object-glass of such power, so 

 divested of both kinds of aberration, and possessing so large 

 an angle of aperture, must be a match for any test object 

 whatever; accordingly, every difficulty at once yields to it, 

 overwhelming efficacy: notwithstanding, I should, myself^ 

 prefer a single triple-glass of the same depth as the combina- 

 tion, at least for opaque objects, which are all within reach 

 of such an angle of aperture as it would be capable of 

 receiving — and such an objective it isin Mr. D.'s contem- 

 plation to make. 



I have, in my paper on aplanatic glasses already alluded 

 to, said much of the extreme difficulty which Mr. Tulley 

 experienced in perfecting his object-glasses, which was won- 

 derfully increased with the reduction of their foci. It will 

 be recollected that his first essay was upon one of nearly as 

 short a focus as 0-3 ; had he commenced with a longer ones 

 doubtless he would have accomplished his purpose far more 

 easily. The chief difficulty he seems to have had to contend 

 with, was the perpetual tendency of his tools (though kept 

 in pairs with steel guages) to alter their figure by some 

 quantity inappreciable by the said guages, yet enough 

 to mar the performance of the object-glass when worked. 

 Mr. D.*8 pre-eminent tact and management have overcome 

 this obstacle, which will of course enable him to work to 

 a regular theory. Whatever may be the merit of over- 

 coming difficulties, that of precluding or exploding them 

 altogether is still greater ; and Mr. Tulley will, I am 

 sure, consider it no disparagement to his own talents to 

 admit, that as Mr. Dollond can work the small aplanatics 

 with certainty and dispatch (which Mr. T. confessedly can- 

 not), the said Mr. D. is the better man, and more con- 

 summate artist, by whom it is no disgrace to be beaten. 



I here commence the proper subject of this paper. 



Length of the body, — It seems adviseable to say some- 



2E2 



