18 yiv, Voiiox 071 Photometry in 



took to discuss, their observations do not call for any further 

 notice in this place. With Professor Forbes, however, I 

 have a much graver controversy. 



In regard to Professor Forbes's ' Memorandum,' we must 

 strive to find his meaning in some curious passages. In 

 speaking of Fresnel's formulae for the intensity of reflected 

 light, he says " scarcely any attempt has been made towards 

 its verification by direct experiment." Before we assent or 

 dissent from the Professor's assertion, we should know what 

 he means by a7i attempt at verification. Attempts, and suc- 

 cessful ones too, have been made to find the quantity of light 

 reflected at various incidences by transparent bodies, by the 

 celebrated Bouguer*, a century ago, and by myself latterly f, 

 in a series of experiments which occupied me several months. 



As Fresnel's formula will not give either Bouguer's quan- 

 tities or mine, our labours must be very unsatisfactory, 

 having done nothing towards that desirable verification. 

 Professor Forbes's paper will probably supply this desidera- 

 tum. A little further on he says, "It occurred to me, about 

 the end of 1837, that the anomalies of photometrical observa- 

 tions being nearly as unsatisfactory as ever, some light might 

 be thrown upon this important subject by ascertaining the law 

 in the case of heat, the intensity of which we have no difficulty 

 in measuring." 



With respect to his actual experiments he says, " I have 

 this winter resumed the subject. I have had an apparatus 

 constructed for securing sufficient accuracy in determining the 

 angle of incidence, and I have used reflecting surfaces, both 

 transparent and metallic; the former are wedges of plate- glass, 

 by means of which reflection from the first surface only may 

 be observed, and the latter are plane specula of steel and silver. 

 The prosecution, however, of these apparently simple experi- 

 ments, has been attended with unforeseen difficulties; and al- 

 though the relative proportions of heat at different angles of 

 incidence are now pretty well determined for glass in several 

 cases, I am not prepared to say whether the absolute amount 

 is exactly the same as Fresnel's formula would give, assigning 

 to heat its proper refractive index. It is satisfactory, how- 

 ever, to know, that the approximation to it is much greater 

 than direct photometrical measures have yet given, with the 

 single exception of two experiments of M. Arago already re- 

 ferred to; and that I have reason to believe that the experi- 

 mental law which Mr. Potter has given from direct observa- 

 tion in the case of light, represents my results much less ac- 

 curately than the theory of Fresnel." This undulatory pas- 



* See Bouguer's Traite d'Optique or Priestley's History of Vision. 

 •)• See Edinburgh Journal of Science for 1830. 



