in the Spectrum." By G. G. Stokes, Esq., M.A., Fellow of Pem- 

 broke College, Cambridge. Communicated by the Rev. Baden Powell, 

 M.A., F.R.S. 



The principal object of the author in this communication is to 

 point out some practical applications of the interference bands re- 

 cently discovered by Professor Powell, the theory of which was con- 

 sidered by the author in the paper to which the present is a supple- 

 ment. The bands seem specially adapted to the determination of 

 the dispersion in media which cannot be procured in sufficient purity 

 to exhibit the fixed lines of the spectrum. The ordinary experi- 

 ments of interference allow of the determination of refractive indices 

 with great precision ; but in attempting to determine in this way the 

 dispersion of the retarding plate employed, there is the want of a 

 definite object to observe in connection with the diff'erent parts of 

 the spectrum. In Professor Powell's experiment, the wire of the 

 telescope, placed in coincidence with one of the fixed lines of the 

 spectrum previously to the insertion of the retarding plate into the 

 fluid, marks tlie place of the fixed line, and so aff'ords a definite ob- 

 ject to observe when the retarding plate is inserted into the fluid, 

 and the spectrum is consequently traversed by bands of interference. 



The practical applications considered by the author are princi- 

 pally four. In the first, the variation of the refractive index of the 

 plate in passing from one fixed line to another is determined, the 

 absolute refractive index for some one fixed line being supposed ac- 

 curately known. The observation consists in counting the number 

 of bands seen between two fixed lines of the spectrum, the frac- 

 tions of a band-interval at the two extremities being measured or 

 estimated. 



In the second application, the absolute refractive index of the plate 

 is determined for some one fixed line of the spectrum. The obser- 

 vation consists in counting the number of bands which move across 

 the wire of the telescope, previously placed in coincidence with the 

 fixed line in question, when the plate is inclined to the incident light. 



The third application is to the determination of the change in the 

 refractive index of the fluid, for any fixed line of the spectrum, pro- 

 duced by a change in the temperature. The observation consists in 

 counting the number of bands which move across the wire of the 

 telescope w'hile the temperature sinks from one observed value to 

 another, the temperature being noted by means of a delicate ther- 

 mometer which remains in the fluid. For this observation a know- 

 ledge of the refractive index of the retarding plate is not required. 



The fourth application is to the determination of the change of 

 velocity of the light corresponding to any fixed line of the spectrum, 

 when the direction of the refracted wave changes with reference to 

 certain fi.xed lines in the plate, which is here supposed to belong to 

 a doubly refracting crystal. The observation consists in counting 

 the bands as they pass the wire when the plate is inclined. It re- 

 quires that the plate should be mounted on a graduated instrument. 

 It would be possible in this way to determine, by observation alone, 

 the wave surface belonging to each fixed line of the spectrum. 



