372 Dr. Wollaston's Method of examining 
ON THE DISPERSION OF LIGHT. 
The method above described for investigating refractive 
powers, may also be employed with similar advantage for 
inquiries into the dispersion of light by different bodies, and the 
consequences that result from their combined action. 
When a glass prism is placed in contact with water, and 
brought near the eye, in such a position that it reflects the light 
from a window, the extent of perfect reflection is seen to be 
bounded by a fringe of the prismatic colours, in the order of 
their refrangibility.* The violet rays, being in this case the 
most refrangible, appear strongest and lowest, on account of 
the less obliquity that is requisite for their reflection. 
But it may happen that two media, which refract unequally 
at the same incidence, may disperse equally at that incidence. 
Under these circumstances, a pencil of rays passing from one 
of such media into the other, will be refracted, without dispersion 
of its colours. The boundary of prismatic reflection would then 
be found a well defined line, free from colour, if the surface at 
which the reflected light emerges from the prism were at right 
angles to its course. 
When the disparity of the dispersive powers of the media is 
still greater, it may also happen, that the usual order of pris- 
matic colours will be reversed; and then the red will appear 
strongest and lowest in the fringe, unless the colours so pro- 
duced are counteracted by refraction at their emergence from 
the prism. 
An instance in which the colours are so reversed, may be 
seen by application of oil of sassafras to a prism of flint glass. 
* Newton’s Optics. Book i. part 2. Exp. 16. 
