452 MATHEMATICS AND NATURAL PHILOSOPHY. 



light. By throwing the compounded velocity of two colours (taking the usual 

 cycloidal type) into a peculiar analytical form, he shows that the resulting vibra- 

 tion will consist partly of a colour whose wave-length is a harmonic mean between 

 those of the original, ;ind partly of irregularities, which may cause the sensation 

 of whiteness, and dilute the colour, and may sometime* be powerful enough to 

 overcome the sensation of colour altogether. This, however, requires that the 

 maximum velocities in each vibration shall Dot be very different from each other, 

 and Professor Challis ingeniously employs this to account for the fact that the 

 mixing of coloured substances produces different results from the mixing of the 

 prismatic colours which to all appearance are identical with the former. The 

 method gives a good explanation of complementary colours, and of several other 

 well-known facts in colour compounds, as given by Newton, Helmholtz, Maxwell, 

 and others. It also leads to the abandonment of the doctrine (always looked on 

 with suspicion) of three primitive colours. There is one point Professor Challis 

 does not notice, which is this : in the composition of two musical notes, whether, by 

 the superposition of vibrations on the vibrating body (as in a string, giving out 

 two notes at the same time), or by their union on entering the ear, each note is 

 still heard separately, and the sensation of harmony is altogether different from 

 the perception of its components; in colour, however, the union of the two may 

 destroy this perception of the components, and give rise to a single sensation 

 only. Now, if we take the same precise analytical forms of vibration in the two 

 cases, it does not appear manifest how this distinction may be made visible in the 

 analytical result. The whole subject is, however, a very difficult one, and, 

 whether altogether sound or not, this idea of Professor Challis is well worth 

 carrying out. 



professor w. Thomson's bakerian lecture (r. s., feb. 28, 1856.) 



This lecture communicates some most valuable discoveries and experiments 

 made by the author in electro thermotics. (1.) An electric current in an 

 unequally heated conductor, if its nominal direction be from hot to cold through 

 the metal, causes a cooling effect in iron and a heating effect in copper. Brass 

 has the same property as copper, and platinum as iron, with respect to this elec- 

 tric conversion of heat. (2.) In thermo-electric inversion between metals, a mode 

 of experimenting is described by which inversions, when they exist, may readily 

 be detected, and the temperature of neutrality determined with precision. 

 Various substances have in this way been subjected to trial by the author. (3.) 

 The effects of mechanical strain, and of magnetisation, on the thermo-electric 

 qualities of metal, are investigated. In a mass of iron under longitudinal stress, 

 the thermo-electric quality across lines of traction differs from that alone/ lines of 

 traction as bars of bismuth differ from bars of antimony. Unstrained iron has 

 intermediate thermo-electric quality between those of the two critical directions 

 under distorting stress. The effect of permanent lateral compression is the same 

 as that of permanent longitudinal extension, or of hardening by wire-drawing, 

 upon the thermo-electric quality of a wire placed lengthwise in an electric circuit 

 in iron, being a deviation from the \ym?~\ constrained metal towards bismuth, and 

 in the other metals (copper, tin, brass, platinum, cadmium and lead [?] ), a devia- 

 tion towards antimony ; also that iu copper and iron, it is the reverse of the 

 effect experienced by the same metal while under the stress that caused the strain. 

 Generally, it is inferred, that in iron hardened by compression iu one direction, 



