TOO TEMPERATURE OF INCANDESCENCE. 



One of tlie most curious relations which as yet 

 have been discovered between light and heat is, that, the 

 temperature at which all bodies become incandescent, 

 excepting such as are phosphorescent, is uniform. The 

 point 011 the thermometer (Fahrenheit's scale) when 

 the eye by perfect repose is enabled to detect the first 

 luminous influence, may probably be regarded as, or 

 very near, 1000. Daniel has fixed this point at 980, 

 Wedgwood at 947, and Draper at 977.* Dr. Eobiiison 

 and Dr. Draper, by independent observations, have 

 both arrived at the conclusion, that the first gleam of 

 light which appears from heated platina is not red, but of 

 a lavender gray, the same in character of colour as that 

 detected by Sir John Herschel among the most refran- 

 gible rays of the solar spectrum, f 



It must be admitted, that the question of the identity, 

 or otherwise, of light and radiant heat, is beset with 

 difficulties. Many of their phenomena are very similar 

 many of their modes of action are alike : they are 

 often found as allied agencies ; but they as frequently 

 exhibit extreme diversity of action, and they may be 

 separated from each other. 



We have now examined the physical conditions and 



* For Dr. Draper's paper, see Philosophical Magazine for May,. 

 1847, vol. xxx. 3rd series. 



| Ott the ^.cf ion of the Rays of the Solar Spectrum on Vegetable 

 Colours : by Sir J. F. W. Herschel, Bait. 



The proof of the continuation of the visible prismatic spectrum 

 beyond the extreme violet may be witnessed in the following 

 manner: " Paper stained with tincture of turmeric is of a yellow 

 colour; and, in consequence, the spectrum thrown in it, if ex- 

 posed in open daylight, is considerably affected in its apparent 

 colours, the blue portion appearing violet, and tbe violet very 

 pale and faint ; but beyond the region occupied by the violet 

 rays, is distinctly to be seen a faint prolongation of the spectrum, 

 terminated laterally, like the rest of it, by straight and sharp out- 

 lines, and which, in this case, affects the eye with the sensation of 

 a pale yellow colour." Philosophical Transactions^ p. 133. 



