212 Dr Brewster on the reflection and decomposition of 



I was now anxious to observe the effect of an approximation 

 to this last condition, or to a perfect equilibrium of all the for- 

 ces which affect the incident rays ; as it is often in extreme 

 cases, and at a limit such as this, that nature delights in the 

 developement of new phenomena. This experiment, however, 

 was attended with more difficulty than I expected ; but amid 

 the numerous disappointments which it occasioned, I was led 

 to the results which I shall now proceed to describe. 



The solids which I employed were two prisms of plate glass, 

 which I shall call A and B. The prism A, whose section 

 was an isosceles right-angled triangle, had its base polished at 

 the plate-glass manufactory where it was made. The prism 

 B was executed for me by Dollond, and very finely polished, 

 having also its section a right-angled isosceles triangle. The 

 refractive indices were 



In A . . . W zr: 1.508 

 In B ... 772 = 1.510 



The fluids which I employed were castor oil and balsam of 

 capivi, the latter having a greater and the former a less refrac- 

 tive power than the glass prisms. The refractive indices were 



In castor oil wi = 1.490 



In balsam of capivi . . . m = 1.528 

 The prisms A, B were now fixed together as in Plate 

 III. Fig. 1, and a film C D of castor oil interposed between 

 them. A ray of light Rr will after refraction atr be reflected 

 in the direction o qm from the surface C o D which separates 

 the prism A and the oil ; and another portion of it will be re- 

 flected in the direction ps m from the surface G j9 H which 

 separates the prism B and the oil. In order that the two rays 

 qm, sn may be sufficiently separated, the common sections of 

 the faces which contain the right angle are slightly inclined 

 to each other. 



When the angle of imcidence Rr E is very great, the light 

 suffers total reflection at the surface C o D. Within the limit 

 of total reflection the light o q m is yellow ; and by diminish- 

 ing the angle of incidence gradually, the pencil o q m passes 

 through all the tints of nearly three orders of colours, as 

 shown in the following Table : — 



