262 Dr. Goring' s Commentary on 



more humble efforts of a mere experimentalist might be rewarded by some 

 useful results." 



It is all vastly proper for a mere tradesman to determine, 

 first of all, the cost of his microscope, and next, the size of the 

 box which is to hold it, having these two points given to find 

 the third, namely, the optical and mechanical construction of 

 his instrument : I have constantly found the spirit of trade 

 not only indifferent, but utterly hostile to all sorts of innova- 

 tions and improvements ; for why should a sh-Optician 

 make them, when they only create trouble without increasing 

 his profits ? A gentleman and a man of science should dis- 

 claim all considerations connected with expense, unless he really 

 means to get up a cheap shop article. I can safely say myself, 

 that I never could have caused any real improvement to have 

 been made in microscopes, unless I had been wholly regardless 

 of expense*. 



A perfect eye is, by a good education and much practice, 

 rendered capable of making very nice distinctions in the per- 

 formance of optical instruments. First-rate opticians will, by 

 merely looking at a brick-wall which they have been accustomed 

 to view, in about a quarter of a minute, be able to look into 

 all the imperfections or merits of a telescope ; yet even they, 

 probably, could not say with certainty, that a lens whose curva- 

 tures are as 2 to 5, had the same aberration as a plano-convex, 

 or that one whose radii were as I to 6, had less (judging only 

 from their j9er/brmance as single object-glasses or magnifiers). 

 The evidence, therefore, of mathematical demonstration is 

 vastly superior to the criterion of the eye, and experiment is 

 only to be resorted to, when there is no theory to guide us ; as 

 was the case, for example, with the triple aplanatic object- 

 glasses for engyscopes f . It is surely no small merit in Mr. W. 

 Tulley, to have discovered effective curves for these by trial, 

 assisted, however, by the theory of those of telescopes. Since I 

 published an account of them in Vol. xxii., p. 265, of this 

 Journal, he has discovered others considerably better ; but it 

 s not probable that he ever will discover, by experiment, the best 



* The iwo first effective achromatics made cost me 80/. 



•y I was not aware that there was any regular theory on the subj'ect when these 

 were first made ; and, at the time this paper was sent to the editor, supposed that 

 the theory of Euler was for double object-glasses like Chevalier's. 



