880 Prof. Challis's Theory of the Composition of Colours 



that if the parts of the eye were acted upon by the setherial un- 

 dulations in exactly the same manner in two individuals, the 

 sensations would be diflferent. Now the undulatory theory of 

 light ought to be capable of giving a reason for every positive 

 sensation perceived by a great number of persons, but is not 

 required to account for the absence of the sensation, or for a 

 different sensation, in a few persons ; because it may be pre- 

 sumed that these exceptional cases are due to causes unconnected 

 with the laws of setherial undulations. 



In the undulatory theory of light, the formula which expresses 

 the physical circumstances corresponding to the sensation of a 

 pure colour is the following, — 



vrsmsmrr- {oe—at + c)f 



in which v is the velocity of a vibrating element of the aether at 

 the distance a; measured parallel to the direction of the motion 

 from a fixed origin, and at the time / reckoned from a given 

 epoch, m is its maximum velocity, a is the velocity of propagation 



of the undulations, — is the time of one vibration backwards and 

 a 



forwards, and c is a constant which is given when the velocity of 



vibration is given at a given position and a given instant. The 



theory assumes that the colour-sensation produced by a simple 



series of undulations depends only on \. 



Hence it may be theoretically inferred that the colours of a 

 pure prismatic spectrum are uncom pounded. For experiment 

 has proved that both the colour and the angle of refraction for 

 a given angle of incidence depend, the substance being given, 

 only on the value of X ; whence it necessarily follows from the 

 above supposition that a ray of definite refrangibility is uncom- 

 pounded. 



This is so strict a deduction from the undulatory theory, that 

 if it should be contradicted by facts, that theory fails in limine. 

 Two kinds of facts have been thought to contradict it ; one de- 

 rived from the composition of colours, the other from phsenomena 

 of absorption. The objections drawn from the former will be 

 met in the course of this communication by showing that the 

 phsenomena of the composition of colours can be explained on 

 the supposition that the colours of the spectrum are pure. At 

 present I will say a few words on the phsenomena of ab- 

 sorption. 



We owe to Sir David Brewster the remarkable discovery, that 

 a ray of a pure spectrum, on entering certain absorbing media 

 which do not allow of its free transmission, may give rise to rays 

 of various colours ; and to Professor Stokes the additional dis- 



