146 MALUS. 



1802). Any substance n, of less refractive power than glass in opti- 

 cal contact with the base of a glass prism m, can be seen by an eye 



at e at any incidence within the limit just mentioned, or while the ray 

 i 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 n 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 - - 1.535 



In the same way Mains 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 



