L 365 3 
XII. A Method of examining refractive and dispersive Powers , 
by prismatic Reflection. By William Hyde Wollaston, M. D. 
F. R.S. 
Read June 24, 1802. 
Xn examining the power with w r hich various substances refract 
and disperse light, I have for some time past employed a me- 
thod unnoticed by writers on optical subjects ; and, as it is not 
only convenient in common cases of refraction, but also capable 
of affording results not attainable by other means, I have been 
induced to draw up a short account of the method itself, and of 
the most remarkable instances of its application. 
This method was suggested by a consideration of Sir Isaac 
Newton's prismatic eye-glass, the principle of which depends 
on the reflection of light at the inner surface of a dense re- 
fracting medium. 
Since the range of inclination within which total reflection 
takes place, depends not only on the density of the reflecting 
prism, but also on the rarity of the medium adjacent to it, the 
extent of that range varies with the difference of the densities 
of the two media. When, therefore, the refractive power of one 
medium is known, that of any rarer medium may be learned,. 
by examining at what angle a ray of light will be reflected" 
from it. 
For instance, when any object is laid under a prism of flint- 
glass, with air alone interposed, the internal angle of incidence at 
which the visual ray begins to be totally reflected, and at which 
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