46 EEPOKT ON PHYSICAL OPTICS. 



them materially in their frequency * But these important and 

 interesting speculations, it must be remembered, are advanced by 

 their author solely with the view of removing an imagined incon- 

 sistency between the phenomena of absorption and the mechanical 

 laws of vibratory movement. We are still far from a precise 

 theory of absorption. When such a theory shall have been estab- 

 lished, there seems reason to believe that it will bring with it also 

 an insight into the internal constitution of bodies even yet more 

 close than that afforded by the affections of polarized light ; and 

 that the laws of molecular action may perhaps, at some future day, 

 be studied in the phenomena of transmitted light. 



The properties of solar phosphori, which attracted so much of 

 the attention of experimental philosophers of the last century, 

 seem at first view to favour the account of absorption suggested 

 by the theory of emission, and to arise from the disengagement of 

 the light which had become united to the body. Canton observed 

 that light may remain in these bodies, as it were in a latent state, 

 for several months, until its re-emission is determined by the action 

 of heat. But it must be observed, in the first place, that the 

 feeble light emitted from the phosphori bears a very small propor- 

 tion to that which they are supposed to receive by absorption. 

 Dessaignes has remarked that most of these substances emit the 

 same kind of light, whatever be the species of light to which they 

 have been exposed.! The same fact has been observed by M. 

 Grotthouss+ and other subsequent inquirers ; and in some of the 

 diamonds possessing the property of phosphorescence, the most 

 efficacious exciting light is of a different colour from that excited. 

 These facts seem to be inexplicable in the theory of emission. In 



* An interesting interference experiment, similar in some respects to that indicated 

 by Sir John Herschel in this paper, has been recently made by Mr. Kane. A com- 

 pound tube, whose branches of 9 and 13 J inches united at the two extremities, was 

 made to sound by the languette of an organ pipe. Each of the tubes, separately, gave 

 its own fundamental note, and all its harmonics ; and when a free communication was 

 opened between them, the system gave all the notes of the two series, with the excep- 

 tion of those whose waves were in complete discordance. Thus, the fundamental note 

 of the short tube was stopped altogether, while its octave was given with remarkable 

 clearness ; the two waves being in complete discordance in the former case, and in 

 complete accordance in the latter. 



t Mem. Inst, torn. xi. 



Journal, 18 15. The same observer discovered the curious fact, 

 that the electric current restored the property of phosphorescence, in many cases 

 where it appeared to have been destroyed by the action of violent heat. 



