light at the surfaces of different media. 21 5 



2d Order. ^ 



3d Order. < 



Having ascertained that at a temperature of about 94° the 

 mean refractive index of the balsam was nearly equal to that 

 of the glass prisms, I proceeded to examine the influence which 

 a varying temperature from 50° to above 94° exercised over 

 the intensity and the colour of the reflected pencil. 



The prisms were therefore fixed so as to exhibit the full 

 blue of the second order, and the heat was gradually applied. 

 The colour of the tint was obviously improved by heat, 

 though the intensity of its light was diminished. No particu- 

 lar change marked the instant when the refractive density of 

 the glass and the balsam was equal. Beyond 94° the inten- 

 sity of the tints increased in consequence of the diminution in 

 the refractive power of the balsam ; but when the temperature 

 was considerably augmented, the tints completely disappeared. 



Let us now attend to a very remarkable phenomenon exhi- 

 bited in the relative intensities of the pencils o q m and p s n. 

 At an angle of incidence of 61° 54' on the surface CoD, and 

 at a temperature of about 50°, the pencil o g m is a full bl ue, 

 while p s n\s a. grayish white of rather less intensity than the 

 blue pencil. By increasing the angle of incidence, the pencil 

 o qm increases rapidly in intensity, white the gray pencil di- 

 minishes slowly : so that at an incidence of 74° o q mis ten 

 or twelve times more luminous than p s n ; whereas at smal- 

 ler incidences than 61° 54', the pencil p s n surpasses oqm'm 



