20 Mr. Potter 07i PJiotometry in 



Calculations from Fresnel's formula from Sir David Brew- 

 ster's paper, Phil. Trans. 1830 : 



Incidence. Crown glass, reflects, of every 100 rays. 



40° 4-910 



45° 5-366 



'50° 6-136 



It appears from the above that Dr. Faraday's experiments, 

 making every allowance for different kinds of glass being used 

 in the different experiments, do not yield that desirable veri- 

 fication which Professor Forbes found amongst his results. 

 However it will be seen that we are not to seek for experi- 

 menta criicis at the above angles of incidence, or at any rate, 

 with such low refracting substances. Dr. Faraday's experi- 

 ments with the heavy optical glasses would have furnished 

 them if we had had the refractive indices of those glasses; for 

 in hiiih refracting bodies the discordance of Fresnel's formula 



... 



with experiments is palpable, for it gives results frequently 

 one-half more, to twice as much as experiment. 



In the Professor's next sentence I have to complain of a 

 want of candour. My first introduction to physical optics 

 was the discovery by experiments, of the law of reflection by 

 metals, of a nature quite different from the suppositions of the 

 scientific world, as well as of myself previously. Sir Isaac 

 Newton had stated his opinion, such as had been received 

 ever since ; Bouguer most likely had it presented to him in 

 his experiments with mercury, but blinded by the commonly 

 received theory, could not see it: for he complains, that after 

 all his precautions the fine dust fell so quickly on his mercury 

 as to hinder the reflection at the higher incidences. Sir 

 William Herschel had examined his splendid specula by 

 photometry without falling upon it. Is it to be wondered at, 

 that I should feel greatly proud of this discovery, my first 

 effort in physical optics ? 



Professor Forbes says, " With respect to reflection at the 

 metals, I believe I may assert that I have verified the remark 

 of Mr. Potter, that metallic reflection is less intense at the 

 higher angles of incidence. I have attempted to ascertain 

 whether it reaches a minimum, and then increases up to 90*^ 

 of incidence, as Mr. Maccullagh supposes; but I have not 

 obtained decisive results. The quantity of heat reflected by 

 the metals is so much greater than Mr. Potter's estimate for 

 light, as to lead me to suspect that his photometric ratios are 

 all too small, which would nearly account for their deviation 

 from Fresnel's law." 



Here my discovery comes to be merely a " remark." I 



