on some Applications of it to important Optical Phmiomena. 1 79 



face to perform an accurate experiment with the comparative 

 photometer. 



For this purpose it is necessary to remove one of the pieces 

 of crown-glass of the former experiment, and to place in lieu 

 of it the substance to be examined, which lias been before 

 properly mounted, and then to find the incidence at which 

 a similar surface on the piece of crown-glass gives an equal 

 reflection. In this manner the larger facet of the diamond in 

 a ring gave me the following results. The results in the first 

 Table I obtained before the instrument was well adapted to 

 the purpose ; those in the second, which were obtained after- 

 wards, I consider to be more correct. 



I. 



II. 



These results are important, as we may compare them with 

 the formula which has the uniform approbation of those who 

 adopt the undulatory theory of light. The unanimous con- 

 clusion of Dr. Young, M. Poisson, and M. Fresnel, who have 

 each investigated the subject, was, that the intensity of the 

 perpendicular reflection, according to that theory, should be 



equal to \—,~-) ', and knowing the value of ju.', or the re- 



fractive index for diamond, we find that this reflection ought, 

 if the undulatory theoiy were true, to be about the double of 



2 A2 



