jllt lloyal Society. 



definite object to observe in connection with the different 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 affords 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 oljser- 

 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 while 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 fixed lines in the plate, which is here supposed to belong to 

 a doubly refracting crystal. The observation consists in counting 

 the bauds 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. 



While considering the theory of Professor Powell's bands, the au- 

 thor was led to perceive the explanation of certain bands, described 

 by Professor Powell, which are seen in the secondary spectrum formed 

 by two prisms which produce a partial achromatism. Although the 

 account of these bands has been published many years, they do not 

 seem hitherto to have attracted attention. It is easily shown by com- 

 mon optics that when two colours are united by means of two prisms, 

 the deviation, regarded as a function of the refractive index, the 

 angle of incidence being given, is a maximum or minimum for some 

 intermediate colour. For the latter colour, two portions of light of 

 consecutive degrees of refrangibility come out parallel ; and there- 

 fore the diffraction bands belonging to different kinds of light, of 

 very nearly the same refrangibility with the one in question, are su- 



