Mr. Potter on Photometty in connexio7i mill Physical Optics. 1 7 



It is thus that almost any analysis, which professed to sup- 

 port the undulator}^ theory of light, has been hailed as a mag- 

 nificent achievement without its minute bearings being ever 

 looked into, provided the general case showed a prima facie 

 accordance with some known facts. In no place has this been 

 more prominent than in the Cambridge Philosophical Society. 



He is a young man and inexperienced who has so little 

 knowledge of human nature as to suppose that in this state of 

 matters any scientific truth, of however important a bearing, 

 which did not fall in with the popular and fashionable prepos- 

 sessions, must be decried, and its discoverer would naturally 

 be held out as a factious and refractory pretender in the re- 

 public of science. 



The scientific public must sooner or later awaken and per- 

 ceive the liberty which has been taken with its confidence, 

 and there must sooner or later arise a time of return to sound 

 philosophizing in physical optics, when the maxims of Lord 

 Bacon will be acknowledged as the only sure guide. 



My objections to Fresnel's formula for the intensity of light 

 reflected and transmitted by transparent bodies, although 

 founded on laborious and careful experimental researches, 

 have been treated as though other men's guesses were more 

 worth than my experiments. I shall, however, before I close 

 this paper, bring corroborative evidence of the accuracy of 

 my results which fortunately exists in print amongst the la- 

 bours of Bouguer and of Sir William Herschel. Dr. Faraday 

 also has given some photometric measures in his Bakerian lec- 

 ture on the manufacture of glass for optical purposes. If the 

 refractive indices of the heavy optical glass had been given, we 

 should have had a good test of Fresnel's formulae from the 

 experiments with those glasses. The angle of incidence which 

 Dr. Faraday employed does not furnish an experimentum crucis 

 for common glass. 



The subject of photometry has been discussed by Professor 

 Lloyd in his report on Physical Optics, read before the 

 ' British Association for the Advancement of Science;' by 

 Professor Powell, in a paper read before the Newcastle meet- 

 ing of the Association, and lately, by Professor Forbes, in a 

 paper read before the Royal Society of Edinburgh, of which 

 an abstract has been printed under the title "Memorandum 

 on the Intensity of reflected Light and Heat. By Professor 

 Forbes. (From the Proceedings of the Royal Society of 

 Edinburgh, March 18, 1839)."'* 



As Professors Lloyd and Powell did not think it necessary 



to make themselves acquainted v/ith the subject they under- 



[* Inserted in L. and E. Phil. Mag. for December 1839, vol. xv. p. 479.] 



Phil. Mag. S. 3. Vol. 16. No. 100. Jan. 1840. C 



