66 Dr Goring on Monochromatic Light, <$fc 



it would have no external focus in front of the bottom glass ; 

 therefore, an achromatic engyscope can only have a double 

 compensation in the same way as a telescope has it, of which 

 latter instrument it is nearly a modification ; and as it is ex- 

 ceedingly difficult to procure perfect flint-glass lenses of large 

 diameters for the object-glasses of telescopes, it is devoutly to 

 be wished, that, by the omnipotence of analysis, we may at 

 length be enabled to construct them absolutely achromatic, of 

 one kind of glass only, by adjusting a system of lenses to par- 

 ticular intervals ; while, with proper attention to the forms of 

 the lenses, their spherical aberration ought at least to be re- 

 duced to insensible quantities, (using only a small angle of 

 aperture,) though it could not perhaps be wholly removed. 



Now, on comparing these propositions with those contained 

 in Mr Coddington's work, under the head of " achromatism ," 

 page 262 to 269, it will be seen how completely I have been 

 compelled to leave him on the left hand. It will be a memo- 

 rable instance of the fallibility of analysis, and perhaps the first 

 on record if the said theories are found to be at fault. I shall 

 leave it to others, however, to draw conclusions and weigh pro- 

 babilities against each other. I have always myself found that 

 good sound theories will produce instruments which satisfy the 

 eye ; and though it must be admitted that this organ is exceed- 

 ingly coarse and easily pleased in the generality of mankind, it is 

 nevertheless in many instances capable of acquiring a refined 

 and accurate taste in optical instruments by a good education, 

 just as it does in pictures and other works of art. In the second 

 volume of Mr Hume's essays, section 12, entitled, " of the 

 academical or sceptical philosophy ,*" part second, there are some 

 curious reasonings on the subject of mathematical demonstra- 

 tion, which the reader would do well to consult. 



I may observe, that Mr Coddington has been at the pains of 

 falsifying his own theories, practically at least, to the best of 

 his abilities, by presenting to opticians a compound micro- 

 scope, termed achromatic, which is constructed according to 

 them. It is described in his treatise on the " Eye and Optical 

 Instruments" pages 60 and 61, under the head of Achroma- 

 tic Object-glass, and is represented in plate 13, figure 190. 

 He has implicated articles 205 and 210 with the expressions 

 in pages 261 and 263, in the treatise on the " Refraction and 



