1886.] 



On the Intensity of Reflected Light. 



275 



IV. " On the Intensity of Light reflected from certain Surfaces 

 at nearly Perpendicular Incidence." By Lord Rayleigh, 

 M.A., D.C.L., Sec. R.S. Received October 6, 1886. 



[Plate 3.J 



In the present communication I propose to give an account of a 

 photometric arrangement presenting some novel features, and of 

 some results found by means of it for the reflecting power of glass 

 and silver surfaces. My attention was drawn to the subject by 

 an able paper of Professor Rood,* who, in giving some results of a 

 photometric method, comments upon the lack of attention bestowed 

 by experimentalists upon the verification, or otherwise, of Fresnel's 

 formulae for the reflection of light at the bounding surfaces of trans- 

 parent media. It is true that polarimetric observations have been 

 made of the ratio of the intensities with which the two polarised 

 components are reflected ; but even if we suppose (as is hardly the 

 case) that these measurements are altogether confirmatory of Fresnel's 

 formulae, the question remains open as to whether the actual intensity 

 of each component is adequately represented. This doubt would be 

 set at rest, were it shown that Young's formulee for perpendicular 



incidence (to which Fresnel's reduce), viz., (?~~^~\ , agrees with 



V+l/ 



experiment. 



Professor Rood's observations relate to the effect of a plate of glass 

 when interposed in the course of the light. He measures, in fact, 

 the transmission of light by the plate, and not directly the reflection. 

 No one is in a better position than myself for appreciating the advan- 

 tages of this course from the point of view of experiment. In the 

 first place, the incidence can easily be made strictly perpendicular, in 

 which case no question arises of a separate treatment of the two 

 polarised components of ordinary light. And, what is much more 

 important, the interposition of the plate leaves the course of the 

 light unchanged, and thus allows the alteration of intensity to be 

 determined in an accurate manner with the simplest arrangements. 



On the other hand, the measurement of the transmitted, instead 

 of the reflected light, is open to grave objection on more than one 

 ground. It may be doubted whether the influence of absorption is 

 altogether negligible, even when the thickness of the plate is as 

 small as that mentioned by Professor Rood, viz., 1'67 mm. But the 

 feature which strikes me most unfavourably is the necessary magnifi- 

 cation of error, when we deduce the proportion of light reflected 



* 'Amer. Journ. Sci.,' vol. 49, 1870 (March) ; vol. 50, 1870 (July). 



