218 Dr Brewster on the reflection and decomposition of 



As nothing depends on the numerical accuracy of these in- 

 dices, I did not measure them with any pecuhar attention ; 

 but by immersing a right angle of each prism in a vessel con- 

 taining each of the three oils, I carefully determined that, at a 

 temperature of 50°, they acted on the homogeneous yellow 

 light of a monochromatic lamp, in the order in which they are 

 above placed. 



I now combined each of the oils in succession with the two 

 prisms, as shown in Fig. 1, and in all the combinations the se- 

 parating surface of the prism A and the oils produced from a 

 white flame, nearly three orders of colours of the same inten- 

 sity, and nearly at the same angles of incidence, as in balsam 

 of capivi ; while the separating surface of the prism B and the 

 oils reflected . only a faint gray image of very little intensity, 

 and generally growing fainter as the angle of incidence in- 

 creased. 



When the homogeneous yellow light of a monochromatic 

 lamp was used, the separating surface of the prism A and all 

 the oils produced the first minimum at nearly the same angle 

 of incidence ; and though I applied heat gradually to the least 

 refractive oil, and cold to the most refractive one, so as to pro- 

 duce a perfect compensation of opposite refractions for the yel- 

 low rays, yet no perceptible change appeared either in the 

 place of the first minimum or in the intensity of the reflected 

 light. In the case of the mixed oil the compensation was ef- 

 fected without any other change of temperature but what was 

 occasioned by a change of position in the apartment. 



In the expectation of discovering some solid or fluid me- 

 dium which would produce with plate glass a greater number 

 of orders of colours, I made the experiments contained in the 

 following tables. 



Table, Showing the periods of colours produced at the se- 

 parating surfaces of plate glass and oils and other fluids. 



Image at the 

 Image at the Surface of Prism A. Surface of Prism B. 



Oil of Cassia. — Pale red tints at 65° 



of incidence ; then at less incidences White and bright, 

 pale blue, and then pale red. Heat 

 strengthens the tints a little. 



