AND THE THEORY OF LIGHT. 6 



metrical knowledge — because, for example, an angle is 

 found to be the measure of force in one instance it is 

 concluded to be the measure of force in others. The laws 

 deducible from the phenomena of the solar spectrum are 

 of this description, and they are certainly very remark- 

 able ; but in their application we are constantly compelled 

 to introduce emendations and corrections, to depart from 

 true logical inferences, and to adopt suppositions to satisfy 

 the conditions of other physical problems. 



3. I make these observations because I find that how- 

 ever easily ordinary minds are satisfied with the theory of 

 prismatic refraction, all scientific investigators have more 

 or less felt the difficulty of the subject. Neither Sir David 

 Brewster nor Sir John Herschel, as far as I understand 

 them, implicitly acquiesces in the doctrine of Newton, and 

 these are as high authorities on the subject as can be 

 obtained. 



The author of the article " Undulations" in the Penny 

 Cyclopcedia concludes in these words : " Much stress is 

 laid on the accuracy with which the phenomena of diffrac- 

 tion are accounted for on the undulatory hypothesis ; but 

 while there yet remains unexplained by that hypothesis so 

 important a circumstance as the different refrangibilities 

 of light, which are satisfactorily accounted for on the cor- 

 puscular theory, and while our knowledge of the action of 

 material particles on one another, as well as of the pro- 

 pagation of motion through elastic media is so imperfect, 

 philosophers seem to be fully justified in suspending their 

 judgment concerning the relative merits of the two rival 

 systems.^' 



4. The experiments on light which I am about to explain 

 lead to other conclusions than have hitherto been obtained 

 from the study of the prism. The data are different, and 

 reasoning from the data the conclusions may be expected 

 to be different. It is only by searching for other data 



