Mr. G. Dodd on 353 



many views and suggestions which merit careful considera- 

 tion, inasmuch as they clash in many points with opinions 

 which have received the stamp of age, and the support of 

 great names. 



This opposition to received opinions, so far from being 

 viewed with distrust, ought to obtain for the writer a larger 

 share of attention, as, in science, it is almost always neces- 

 sary, before we can make real additions to our knowledge, 

 to eradicate some previous notions, either ill formed or 

 incomplete. 



The principles endeavoured to be established by Mr. 

 Cooper, appear to be these : — 



1. That White Light consists of three colours , Red, Green, 

 and Violet. 



2. That there are but three indices of refraction in any given 

 substance, viz., one for each colour. 



3. That the two preceding principles are sufficient to explain 

 the phenomena of diffraction. 



4. That, supposing the 2nd proposition to be correct, the 

 dark intervals which would necessarily residt in the prismatic 

 spectrum, may probably be filled up by multiplied reflexion 

 within the body of the prism. 



It is not my intention, at present, to offer any remarks 

 on the general nature of the experiments contained in 

 Mr. Cooper's papers, nor upon the evidence which he 

 adduces in support of the three first propositions ; I will 

 merely confine my attention to the 4th, with a view to 

 draw Mr. Cooper's notice to a few circumstances, which 

 seem to militate against the correctness of his deduction. 



The extent to which multiplied reflexion can be carried 

 in the body of a prism, without exhausting the luminosity 

 of the ray, appears never to have been made the subject of 

 experiment ; but I will endeavour to show, that be that 

 extent great or small, it can never produce the effect sur- 

 mised by Mr. Cooper. 



In order the more clearly to shew this, I will at once 

 assume, for the sake of argument, that the two first pro- 

 positions are true : that white light consists of red, green, 

 and violet, and that each colour has but one index of re- 

 fraction : I will then detail reasons for believing that the 



VOL. IV. 2 A 



