1883.J 



Some Experiments on Metallic Reflection. 



41 



The adamantine property of a substance, as for the sake of a name 

 it may be called, increasing on the whole with the index, and conse- 

 quently on the whole with the density, there can be little doubt that 

 if metals, retaining their actual density, were transparent like diamond, 

 they would exhibit this property in a greatly exalted degree. On the 

 other hand, the metallic properties connected with intense absorption 

 are exhibited by many non- metallic substances, such for example as 

 the colouring matters derived from aniline. Hence we have very 

 strong reason for believing that there are two distinct and independent 

 properties in a metal by virtue of either of which, if it stood alone, 

 there would be a change of phase and persistence of intensity for light 

 polarised perpendicularly to the plane of incidence as the angle of 

 incidence was increased. The changes due to these two would not 

 follow the same laws as regards their dependence on the angle of 

 incidence. The coefficients expressing these two properties, together 

 with what answers to the index of a transparent substance, make 

 three physical constants which are required to define a metal optically, 

 even in relation to homogeneous light. 



If we confine our attention to an experimental determination of 

 the difference of phase and ratio of intensity for light polarised in and 

 perpendicularly to the plane of incidence, and if, neglecting altogether 

 the adamantine property, we determine the two constants in the 

 ordinary formulae for metallic reflection so as to make them agree 

 with observation in two cases, suppose by giving a difference of phase 

 of 90° for an observed angle, and an observed ratio of intensities at 

 that angle, it may be readily imagined that the ordinary formula? may 

 be to a certain extent formulae of interpolation, giving the difference 

 of phases and ratio of intensities for other angles of incidence without 

 any very material error ; and yet when we come to a totally different 

 kind of observation, such as that of determining the ratio of the 

 intensity of incident to that of reflected light, that the formula? may 

 be found to be very distinctly in error. Hence observations of this 

 latter class seem deserving of more attention than have lately been 

 bestowed upon them, lest from too great reliance on the accordance 

 between theory and observation as regards the difference of phase and 

 ratio of intensities when we compare light polarised in and per- 

 pendicularly to the plane of incidence, we should be led unduly to 

 trust the formula?, for giving correctly the ratio of intensities for 

 incident and reflected light. — Gr.G.S. 



