DR. BREWSTER ON REFLECTED LIGHT. 



193 



Colours. 



Angles of Incidence 

 RrE. 



f-Bliiishpink 28 



Full blue 26 



Bluish green 22 



Bluish yellow 18 



2nd Order.^ Yellow 10 



Reddish yellow 1 



Red . — 8 



Pink red —13 



-Limit of pink and blue . . . . — 16 



"Blue —22 



Bluish green —26 



Green —30 



^Yellowish green —41 



3rd Order.^; 



Angles of Incidence 

 on the Surface 

 CoO. 

 o / 



. 63 8 



. 61 54 



. 59 23 



. 56 50 



. 51 37 



. 45 40 



. 39 42 



. 36 25 



. 34 28 



. 30 37 



. 28 56 



. 25 29 



. 19 13 



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 ob- 

 viously improved by heat, though the intensity of its light was diminished. 

 No particular change marked the instant when the refractive density of the 

 glass and the balsam was equal. Beyond 94° the intensity of the tints in- 

 creased 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 phaenomenon exhibited in the rela- 

 tive intensities of the pencils o qm and p s n. At an angle of incidence of 61° 54' 

 on the surface C o D, and at a temperature of about 50°, the pencil oqmisdi full 

 blue, while jo 5 w is a grayish white of rather less intensity than the blue pencil. 

 By increasing the angle of incidence, the pencil oqm increases rapidly in inten- 

 sity, while the gray pencil diminishes slowly : so that at an incidence of 74° 



MDCCCXXIX. 2 c 



