MEMOIR VII. ] STUDIES IN THE DIFFRACTION SP 



> V x 



Newton's principles the particles of light shcmSl motfe 

 faster through water than through air ; on the theory of A 

 Huyghens, waves of light must move slower in wafer 

 than in air. The experiments of the French physicist^ 

 proved that the latter is the case. This may, 

 be considered as the successful establishment of the un- 

 dulatory theory. It has, moreover, given that strik- 

 ing proof of its truth which may be considered as the 

 criterion of any theory the ability to foretell results. 

 This it did in the case of the discovery of conical re- 

 fraction. 



Light, therefore, consists in the transference of energy 

 or force, not in the transference of matter. 



The grating I employed in the experiments hereinafter 

 related was made for me by Mr. Saxton, at the United 

 States Mint in Philadelphia, more than thirty years ago. 

 Though from the work it did for me I cannot but speak 

 of it with admiration it enabled me to make the first 

 photograph that was ever executed of the diffraction 

 spectrum yet it was far from being equal to the mag- 

 nificent ones of Mr. Rutherfurd, This grating was five 

 eighths of an inch long and one third of an inch in 

 breadth. Mr. Rutherfurd's gratings have in some speci- 

 mens 17,240 lines to the inch. I had found previously 

 to 1843 that it is more advantageous practically to use a 

 reflecting than a transparent grating, arid accordingly I 

 silvered mine with mercury-tin amalgam, such as is used 

 in ordinary looking-glasses. Mr. Rutherfurd's reflecting 

 gratings are coated with pure silver, by an operation 

 more recently discovered. 



I will now relate the use of these gratings, and de- 

 scribe some of the important discoveries made by them. 



Let a beam of light, S A', Fig. 11, pass through a nar- 

 row slit, S, and fall perpendicularly on the ruled grating, 



