1 1+ Mr. Talbot's Facts relating to Optical Science. 



Remark. — I have used the revolving mirror for many years 

 as an instrument of optical and photometrical research ; but 

 the idea of applying it to determine the velocity of electricity 

 belongs entirely to Mr. Wheatstone. The electrical experi- 

 ment which I proposed in this Journal was distinctly stated to 

 be founded upon his experiment exhibited at the Royal Insti- 

 tution, only proposed to be tried upon a much larger scale, 

 (viz. with a wire of some miles in length,) but since Mr. 

 Wheatstone says in his letter * (which I have only lately seen, 

 having been abroad for some months,) that he had already 

 tried an experiment upon the same principle, I most willingly 

 yield to him the priority of it. It is a trifle, not worth a mo- 

 ment's hesitation. 



5. On the Flame of Lithia. 

 Lithia and strontia are two bodies characterized by the fine 

 red tint which they communicate to flame. The former of 

 these is very rare, and I was indebted to my friend Mr. Fara- 

 day for the specimen which I subjected to prismatic analysis. 

 Now it is difficult to distinguish the lithia red from the stron- 

 tia red by the unassisted eye. But the prism displays be- 

 tween them the most marked distinction that can be imagined. 

 The strontia flame exhibits a great number of red rays well 

 separated from each other by dark intervals, not to mention 

 an orange, and a very definite bright blue ray. The lithia 

 exhibits one single red ray. Hence I hesitate not to say that 

 optical analysis can distinguish the minutest portions of these 

 two substances from each other with as much certainty, if not 

 more, than any other known method. 



6. On the Flame of Cyanogen. 

 For the opportunity of examining the optical characters of 

 this flame, I am likewise obliged to the kindness of Mr. Fara- 

 day, who showed it both to Sir J. Herschel and myself at the 

 Royal Institution. When viewed with a prism, this flame 

 presents a very distinct and peculiar character, separating the 

 violet end of the spectrum into three portions, with broad dark 

 intervals between. But the most remarkable fact is this, 

 that the last of these portions is so widely separated from the 

 others as to induce a suspicion that it may be more refracted 

 than any rays in the solar spectrum, a question which I should 

 be glad to have the opportunity of deciding by direct experi- 

 ment. This separate portion has a pale undecided hue. I 

 should hardly have called it violet, were it not situated at the 

 violet end of the spectrum. To my eye it had a somewhat 

 whitish or grayish appearance. 



* See Lond. and Edinb. Phil. Mag., vol. iii. p. 204.— Edit. 



