350 Prof. Draper on the production of Light hy Heat. 



low as 1095°. Under these circumstances the total length 

 couhi not be compared by direct measurement with the other 

 observations, and the result given in fig. '1 is from the best 

 judgement I was able to form : the colours were red and 

 greenish-gray. 



The gray rays emitted by platinum just beginning to shine 

 appear to be more intense than the red ; at all events the 

 wires in the field of the telescope are more distinctly seen 

 upon them than upon the other colour. I give them the de- 

 signation of gray, for they appear to approach that tint more 

 closely than any other ; and yet it is to be remarked that they 

 are occupying the position of the yellow and green regions. 



Already we have encountered a fiict of considerable import- 

 ance. The idea, that as the temperature of a body rises it 

 begins to emit rays of increasing refrangibility, has obviously 

 to be taken with a certain restriction. Instead of first the 

 red, then the orange, then the yellow, &c. rays, in succession 

 making their appearance, in which case the spectrum should 

 regularly increase in length as the temperature rises, we here 

 find, at the very first moment it is visible to the eye, it yields 

 a spectrum reaching from the fixed line B to nearly F; that 

 is to say, equal to al)out two- thirds the whole length of the 

 interference spectrum, and almost one-half of the prismatic. 



It is to be remarked, that while the more refrangible end 

 undergoes a great expansion, the other extremity exhibits a 

 corresponding though a less change. As very important 

 theoretical conclusions depend on the proper interpretation of 

 this fact, we must not forget that, to a certain extent, it may 

 be an optical deception, arising from the increased brilliancy 

 of the light. While the rays are yet feeble, the extreme ter- 

 minations may be so faint that the eye cannot detect them ; 

 but as the intensity rises, they become better marked, and an 

 apparent elongation of the spectrum is the consequence. 



It is agreed by optical writers, that to the human eye the 

 yellow is the brightest of the rays. In the prismatic spectrum 

 the true relationship of the colours is not perceived, because 

 the less refrangible are crowded together, and the more re- 

 frangible unduly spread out. But in the interference spectrum, 

 where the colours are arranged side by side in the order of 

 their wave-lengths, the centre is occupied by the most lumi- 

 nous portion of the yellow; and from this point the light de- 

 clines away on one side in the reds, and on the other in the 

 blues, the terminations being equidistant from the centre of 

 the yellow space. 



Now if the rays coming from shining platinum were passed 

 through a piece of glass, on which parallel lines had been 



