146 MALUS. 
1802). Any substance 2, of less refractive power than glass in opti- 
cal contact with the base of a glass prism m, can be seen by an eye 
: ay 
ors 
at e at any incidence within the limit just mentioned, or while the ray 
z entering the other side of the prism and impinging on its base, is in- 
capable of being refracted out at the base, and therefore reflected from 
within; but as soon as this limit is exceeded, or the ray is refracted 
out at the base, then n ceases to be visible at e. The exact incidence 
or “critical angle’’ at which this takes place, is measured by an 
appropriate apparatus, and the refractive index for m deduced, that: of 
the prism being known, a series of substances being applied in suc- 
cession, whether transparent or opaque, Dr. Wollaston in this way 
determined their refractive indices. As the different primary rays 
have indices a little differing, and which are greatest for red light, Dr. 
Young remarked that the limit thus found applies in strictness to the 
extreme red ray. 
In this way Dr. Wollaston found the refractive indices as follows:— 
White wax, boiling = - - - 1.542 
Ditto cold - - _ - 1.5385 
In the same way Malus found 
Wax at 14° Reaum. (=63° Fahr.) - - 1.5123 
Ditto melting . - - 1.4503 
Ditto boiling - - - 1.4416 
(These numbers are all lower than the former, probably from a dif- 
ferent sort of wax being used.) 
Dr. Wollaston, in applying the simple calculation above indicated 
to the observed angles, did not question the very natural assumption, 
that the same formulas would apply to the observed angles equally, 
whether the substance was opaque or transparent, solid or fluid. 
Laplace, in a theoretical investigation founded on certain consider- 
ations derived from the molecular theory, framed his formulas on the 
assumption that the conditions were different for opaque and for 
transparent bodies, and even for the same substances in the two states 
respectively. The question at issue was the truth of this assump- 
tion, though it must be confessed that little appears in the tenth book 
“ 
