268 Prof. Challis on a Theory of the Dispersion of Light. 



the contact between these two metals must show relatively great 

 variations in temperature, without these indicating any consider- 

 able electromotive force between them. 



If the metals are arranged according to the quantities of heat 

 which are absorbed or produced in case a voltaic current traverse 

 the place of contact, it does not seem to me that it is a priori 

 certain that we should obtain the same series as that formed 

 when they are arranged according to their electrical tension on 

 contact. It seems conceivable that the magnitude of the cur- 

 rent which a contact can produce does not depend simply on the 

 tension which, the electricity can attain when the insulated 

 metals are placed in contact, but also on the time necessary 

 for the production of this state. Though this time is certainly 

 very short, it may doubtless be comparable with the time for 

 the passage of the current from one pole to the other. If it 

 is indeed so, the ordinary electrical series for the case in which 

 a real current is produced cannot without further proof be re- 

 garded as the right one. What is the real state of the case must 

 be decided by trustworthy measurements of the heat absorbed 

 and produced. Peltier's phenomena obtain thus an unexpected 

 interest. If time and circumstances permit, I hope before long 

 to make an experimental determination of the quantities of heat 

 in question. 



XXXII. Comparison of a Theory of the Dispersion of Light on the 

 Hypothesis of Undulations with Ditscheiner's determinations of 

 Wave-lengths and corresponding refractive Indices. By Pro- 

 fessor Challis, M.A., F.R.S., F.R.A.S* 



THE Theory of the Dispersion of Light which I proposed in 

 this Journal in 1864 is, I believe, the only one which may 

 be strictly said to rest on the hypothesis of undulations. It was 

 commenced in the Number for June of that year; and in the 

 Supplementary Number for December it is compared with the 

 refractive indices of two substances for seven principal rays, 

 Fraunhofer's values of the wave-lengths of the rays being adopted. 

 At the end of an article on the Undulatory Theory of Light in 

 the Philosophical Magazine for May 1865 the same comparison 



o 



is made by means of Angstrom's values of X for the same rays. 

 The theory is reproduced in my work ' On the Principles of Ma- 

 thematics and Physics ' — at first, just as it was originally pro- 

 posed; but subsequently, while the work was in the press, it 

 occurred to me that a course of reasoning somewhat different in 

 principle would be more exact, and, accordingly, by another in- 



* Communicated bv the Author. 



