1883.] 



Some Experiments on Metallic Reflection. 



29 



The metal mirror was then clamped to the stage, which had pre- 

 vious! y been carefully adjusted, in order that the surface of the mirror 

 might be vertical and in the axis of the instrument, and the whole 

 instrument rotated on its outer axis until the reflected beam fell on 

 the photometer. Four readings were then made, as has already been 

 described, the mirror removed, and four more readings made of the 

 intensity of the light emitted by the lamp ; the mirror replaced, the 

 angle of incidence altered, and four more readings made, and so on. 



It was found necessary to make the observations in this way, as 

 although the two lamps were trimmed as nearly as possible alike, 

 considerable fluctuations in their relative intensity not unfrequently 

 occurred, and in order to diminish as far as possible this source of 

 error, the mean of eight observations, four before and fonr after the 

 measurement of the intensity of the reflected light, was taken as the 

 true intensity of the light incident upon the mirror. 



The metal surfaces used measured abont 8 centims. by 5 centims., 

 were accurately plane, and had all been polished with putty powder, it 

 having been previously ascertained (" Proc. Roy. Soc," vol. 31, 

 p. 486) that the optical constants for metallic surfaces depend to a 

 certain extent upon the nature of the substance with which they have 

 been polished. 



It had been originally intended to use light polarised in and per- 

 pendicularly to the plane of incidence, and to determine the ratio of the 

 reflected to the incident light in either case, but it was found im- 

 possible to make any satisfactory measurements, owing to the great loss 

 of light. The intensity of the light could have been somewhat in- 

 creased by the use of a larger Mcol, but it seemed so very doubtful 

 whether sufficient light would be thus obtained, that it was thought best 

 to abandon the use of polarised light. Ordinary light being equivalent 

 to two beams of light of equal intensity polarised at right angles to 

 each other, if the total amount reflected at any angle, and the ratio of 

 the intensities of the light polarised in and perpendicularly to the 

 plane of incidence when light polarised at an angle of 45° with that 

 plane is incident upon the surface of the plate at the same angle, are 

 known, the reflective power of the plate for light polarised in and 

 perpendicularly to the plane of incidence can of course be readily 

 calculated. 



The table gives a series of measurements made with a silver plate. 

 The numbers in the first column are the distances in centimetres of 

 the sliding lamp from the photometer when the light of the other 

 lamp fell direct on the paper, and those in the third when the light 

 was reflected by the mirror. The means of these observations are 

 contained in the second and fourth columns, the angles of incidence in 

 the fifth column, and the ratios of the reflected to the incident light, 

 the latter being taken as 100, in the sixth column. As the intensity 



