238 Dr Goring"* Remarks on M. Chevalier's Paper, 



Art. IX. — Remarks on Mons. Chevalier's Paper, and on the 

 Memoir of Elder. By Dr Goring. Communicated by the 

 Author. 



A. It will no doubt be thought that M. Chevalier has been 

 somewhat tardy in his reply ; but it must be recollected that, 

 in order to answer me with effect, it was necessary for him to 

 perfect his object-glasses, which he has even now scarcely ac- 

 complished, at least to the extent contemplated by him. 



A A. Simplicity is, in my mind, a capital ingredient in all in- 

 ventions and works of art. M. Chevalier has invented a kind of 

 object-glass which will assuredly descend to posterity, for it is 

 almost the only form in which achromatic object-glasses for en- 

 gyscopes can be made with any sort of facility by moderately 

 gifted artists ; the radii of their inner surfaces being alway ri- 

 gorously the same, generate double aplanatic foci and a variable 

 aberration, on which Mr Lister has founded an admirable sys- 

 tem of correction. These advantages, however, were never 

 contemplated or applied by M. Chevalier, and are the exclu- 

 sive discovery of Mr Lister. 



B. So effectually had M. Chevalier blinded his object-glasses, 

 with respect to lined proof-objects, by an excessive reduction of 

 their aperture, (owing to his ignorance of them at the time,) 

 that had we been disposed to take an ungenerous advantage of 

 his insensibility to the value of aperture, it would have been 

 easy to have utterly destroyed the reputation of his instru- 

 ment, (which could always have spoken for itself,) instead of 

 which, we, with the greatest tenderness, opened the eyes of his 

 unfortunate offspring, and made them see very well. 



C. One of the quadruple combinations of this set was beau- 

 tifully perfect; the sextuple system was well corrected for sphe- 

 rical aberration, but rather over-corrected in point of dispersion. 

 I certainly do not think this defect of much consequence. It 

 merely causes the outline of a transparent object to be of a 

 bluish colour, instead of its natural tint. Under-correction gives 

 an outline of red and orange, which I utterly detest. Neverthe- 

 less, in order that a false doctrine may not be deduced from this 

 concession, which might be found exceedingly comfortable and 

 convenient to slovenly opticians, viz. that if the spherical abcr- 



