1886.] Light reflected at nearly Perpendicular Incidence. 293 



= tan 57° 55' = 1-5952. 



This relates to white light. 



To find the index of refraction by the method of total reflection, 

 the mirror was mounted vertically in a small tank of plate glass, 

 cemented with glue and treacle, and containing bisulphide of carbon. 

 The mirror E, as shown in fig. 2, was parallel to one of the sides of 



the tank, and a cover was provided to check evaporation. A uniform 

 field of homogeneous light could be obtained from a salted spirit 

 lamp, A, with the aid of a plate of ground glass, B, and a collimating 

 lens, C. The eye looking in along such directions as GF, is able to 

 mark with considerable accuracy the direction in which total reflec- 

 tion begins. By the aid of plumb-lines, &c, this direction and that 

 of the face of the mirror (seen from above) were marked upon a 

 board, and it appeared that the angle GLK, between the face of the 

 mirror and the direction GLF of the first totally reflected emergent 

 light was 18°. 



A beautiful variation in the experiment may be made by replacing 

 the spirit lamp with a candle, and subsequently analysing the reflected 

 light by a direct vision prism. For this purpose a screen carrying a 

 slit should be interposed as near F as conveniently may be. As the 

 incidence of the light upon the black glass becomes more grazing 

 total reflection sets in, but first at the violet end of the spectrum. 



Fig. 2. 



