Mr. Coddington's Reply to Dr. Goring. 113 



and speaks of it, with his accustomed modesty, as of a youth- 

 ful production, which has at least done no harm, except, in- 

 deed, as it appears, exhausting Mr. Tully's patience. 



Dr. Goring, confessing that he cannot discover the least 

 flaw in our theories, proposes to oppose them by facts, a prac- 

 tice which I cannot too highly applaud, being most firmly con- 

 vinced that no theory can be considei'ed as worth a farthing, 

 except for the indirect advantage of exercising the intellect, 

 until experience has shown that it is complete and satisfactory. 

 Instead, however, of keeping this promise he proceeds to lay 

 down certain " propositions, which rest only on the basis of 

 the evidence of the eyes or experience." Some of these pro- 

 positions unfortunately my own limited experience enables me 

 most flatly to deny. I will beg leave to quote them in order, 

 with a few remarks. 



1. " When achromatism is obtained by the adjustment of 

 lenses to particular intervals, as in the case of the Huygenian 

 eye-piece, such achromatism is absolute and perfect, and not 

 like that effected by the combination of a concave lens of flint- 

 glass with a convex of plate or crown glass, which never effects 

 a complete neutralization of the chromatic aberration, as is 

 well known." 



The Doctor seems here to confound together two distinct 

 modifications of chromatic dispersion, which affect such instru- 

 ments as a telescope or a microscope quite independently, and 

 must be corrected, if necessary, on quite different principles, 

 as I have endeavoured to explain, after Professors Robison, 

 and Airy. The achromatism is in neither case perfect, and 

 it is impossible to compare them, because they depend, though 

 in different manners, on the apertures of the lenses employed 

 as object-glasses and eye-pieces respectively, which are hardly 

 connected with each other by any laws whatever. 



2. " The only kind of achromatism, produced by convex 

 lenses, which is known in practice, is when two are adjusted 

 to an interval equal to one half the sum of their focal distance 

 or thereabouts." 



Every person who knows anything of the construction of 

 a telescope is well aware that an erecting eye-piece, consisting 

 of four lenses, is, when properly made, just as achromatic as 

 the Huygenian. 



3. " Many modifications of this combination may be made, 

 as by doubling or tripling the eye- and field-glasses." 



This is very true, and I have applied it witii complete suc- 

 cess, in order to correct a defect totally unconnected with 

 chromatic dispersion. 



iV, a. Vol. 10. No. m. Aug. 1831. Q ♦. « III 



