EFFECTS OF HEAT. 389 



To one octave, therefore, its range is limited. The extreme red ray, which 

 is emitted by a substance just becoming red hot at a temperature of 

 1026 Fahr., and which is the least refrangible that can affect the eye, 

 is caused by vibrations that are exactly half as frequent as the extreme 

 violet ray emitted by the sun. It is important, in the explanations we 

 are giving, to understand that, in a perfect solar spectrum, the distribution 

 of the colored spaces is totally different from what it is in the case of the 

 prismatic. In such a spectrum, as produced by the interference of rays 

 passing through a surface of glass on which have been ruled with a point 

 of a diamond parallel lines the yfl-J-g-g- ^ an mcn apart, the yellow occu- 

 pies the middle region, and from this the light grades off, terminating at 

 equal distances with the extreme red on one side and the extreme violet 

 on the other. The circumstances of such an experiment prove that, the 

 wave length for the red light being compared with that for the yellow, 

 and also for that of the violet, they bear to one another the extraordinary 

 and simple relation of 1, 1J, 2, establishing the assertion just made, 

 that the extreme limit of perception of the eye is comprised in a single 

 octave. 



I may refer to the experiments published by myself on this point, and 

 also to those both antecedently and subsequently published 

 by M. Melloni, in proof of the unreliability of the method of colored rays is 

 examining the solar spectrum by the prism in the manner jeinu? 

 introduced by Newton. More particularly to the discussion minating pow- 

 now before us does this remark apply ; for the prism, as er * 

 may be gathered from what has just been said, spreads out the colors of 

 light unduly, and gives false indications respecting the distribution of 

 heat. There can now remain no doubt, although the prism indicates the 

 contrary, that the yellow, or brightest ray of light, is the hottest, and 

 that the warming power of the others, orange, green, &c., follows in the 

 order of their luminous < intensity. When we have finished a descrip- 

 tion of the nervous mechanism of the eye, we shall find that the expla- 

 nation of its function turns on the admission of this fact. 



The eye is limited in another respect ; it can not simultaneously com- 

 pare lights which differ from one another in brilliancy if the Limit in the 

 one should be upward of 64 times as bright as the other. 

 The more luminous overpowers or extinguishes the feebler, of lights. 

 We can not see the light of a candle if we hold it up against the sun. I 

 may again refer to the experiments I have published, establishing that 

 upon this fact is founded the most exact method of photometry yet 

 known. 



2d. Of the Nervous Mechanism of the Eye. 

 In the preceding description it was stated that the retina, commonly 



