34 THE RADIATIONS OF IGNITED BODIES. [MEMOIR I. 



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 color. 

 The designation of gray may be given them, for they ap- 

 pear 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 fact of considerable 

 importance. The conclusion that as the temperature of 

 a body rises it emits rays of increasing refrangibility has 

 obviously to be taken with a certain restriction. Instead 

 of first the red, then the orange, then the yellow rays, 

 etc., in succession making their appearance, in w^hich case 

 the spectrum should regularly increase in length as the 

 temperature rises, we here find that at the very first mo- 

 ment it is visible to the eye it reaches from the fixed 

 line B nearly to F, that is to say, it is equal to about two 

 thirds of the whole length of the diffraction 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 ex- 

 hibits a corresponding though a less change. As very 

 important theoretical conclusions depend on the proper 

 interpretation of this fact, it must not be forgotten 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 terminations may be so 

 faint that the eye cannot detect them, but as the inten- 

 sity rises they become better marked, and an apparent 

 elongation of the spectrum is the consequence. 



It is agreed among optical writers that to the human 

 eye the yellow is the brightest of the rays. In the pris- 

 matic spectrum the true relationship of the colors is not 

 perceived, because the less refrangible are crowded to- 



