MR. POWER ON THE ABSORPTION OF THE SOLAR RAYS, ETC. 
29 
■ sin 0 
the equation 
may, in fact, be derived from that hypothesis, without any con- 
sideration of the spread of the wave. 
To make this appear, suppose PQ, a transverse section 
of the incident beam, to be all in the same phase of vibra- 
tion, and after a certain time to have undulated into the 
position P ; Q ; , having been previously refracted by the 
surface AB. The time from P through A to P ; will be 
PA AP, 
a 1 a l 
that from Q through B to Q, will be 
BG BG, 
a ' a t ' 
Equating these and transposing, we get 
PA-BQ BG-AP. 
— b 
a a { 
that is, drawing QR and P,S parallel to AB, 
and since RQ = AB = P,S, 
But 
and 
PR Q,S 
PR a Q V S 
RG — «/ P,S* 
PR 
RQ 
— cos PRQ= sin 0, 
sin SP,Q ifPOS — 90° 
P,S sin P,G,S Sin ' 1 WP — ^ 
and only on that supposition. 
Hence the equation sin Q=- sin 6 
Cl, 
expresses the condition that P,Q ; S is a right angle, in other words, that the direction 
BQ, of the refracted wave is perpendicular to the section of similar phase to PQ. 
Now though this may be true with regard to sound, we have no reason, beyond a 
precarious analogy, to assume that it is true with regard to light. Indeed I think 
the hypothesis of a similarity of phase extending over the wdiole of a transverse section 
of the ray, whether it be the incident or refracted ray, is quite untenable ; for let us 
consider how light is generated. 
29. Light appears to be generated by the action upon the ether of the superficial 
particles of a vibrating body, whether those vibrations have their origin in the pro- 
cess of combustion, as in the flame of a candle, or in some other way, as in the case 
of phosphorus, the electric light, &c. The vibrations of these superficial particles 
must be performed in that superficies, otherwise they could not impart transverse 
vibrations to the ether in contact with them: and such being the case, it is highly 
