1883.] Some Experiments on Metallic Reflection. 27 



due to the light reflected by the glass appeared equally bright, i.e., 

 till the ordinary image of the light reflected by one-half of the mirror 

 was equal to the extraordinary image of the light reflected by the 

 other half. 



The refractive index of the glass being known, the amount of light 

 reflected by it at any angle could be calculated by Fresnel's formulae, 

 and thus the percentage of light reflected by the metal determined. 



The numbers obtained by M. Jamin agree well with those deduced 

 by calculation from Cauchy's theory,, and also with those experi- 

 mentally determined by MM. De la Provosfcaye and Desains, by 

 means of the thermopile. Unfortunately, however, the experiments 

 on this point made by these two eminent French physicists were not 

 very numerous, and the method used by M. Jamin has been described 

 by M. Verdet (" Lecons d'Optique Physique/' ii, 546) as " TJn 

 procede indirect qui n r est pas susceptible d'une grande perfection." 



Under these circumstances I trust that some experiments which I 

 have recently made on the subject may be thought worthy of pub- 

 lication, although, owing mainly to the difficulties inseparable from 

 all photometric determinations, the observations are not as concordant 

 as could be wished. 



The method used was essentially that of Potter, the experiments 

 being made by comparing photometrically the amount of light 

 reflected by a polished metallic surface at different angles with that 

 which fell directly on the photometer when the reflecting surface was 

 removed. 



This method is, of coarse, only applicable to the white metals, and 

 in order to obtain anything like accurate results the mean of a con- 

 siderable number of observations must be taken. 



Two similar parafline lamps, with flat wicks, were used, one 

 arranged to slide along a horizontal board about two metres long, to 

 which a scale divided into millimetres was attached, and the other 

 supported by a metal ring fastened to one of the arms of a Babinet's 

 goniometer. 



An endless cord, which passed round a pulley with a handle at one 

 end of the board, enabled the first-mentioned lamp to be moved and 

 placed at different distances from the photometer. 



It was originally intended to use a Bunsen's disk, but it was found 

 that owing to the small size of the beam of reflected light, it was not 

 possible to make satisfactory measurements with it, and after various 

 arrangements had been tried, a modification of Ritchie's photometer 

 was finally adopted. Two pieces of white paper were so placed that 

 whilst both were visible to the observer, one being slightly in front 

 of the other and overlapping it to a small extent, each received light 

 from one only of the lamps, and when equally illuminated the edge ot 

 the front paper vanished. Two triangular blocks of wood 4 centims. 



