MEMOIR VII.] STUDIES IN THE DIFFRACTION SPECTRUM. 



things that were fragmentary and confused spontaneous- 

 ly fell into an orderly arrangement. 



While the theory of optics was making this great 

 advance, another important science, physiology, was pre- 

 senting a similar development. It was casting off the 

 Vital Force of the older medical authors, and acknowl- 

 edging the dominion of chemical and physical forces. 

 It had become plain that the interpretation of many 

 phenomena, as hitherto received, must be changed. 



We may apparently have heat without light, and 

 light without heat. In the darkest room we cannot 

 perceive vessels filled with boiling water, yet the warmth 

 we experience on approaching them assures us that they 

 are emitting radiations. Is not this heat without light ? 

 If we stand in the rays of the full moon, we cannot de- 

 tect any increase of temperature. Is this not light with- 

 out heat ? It is true that in this latter instance we are 

 mistaken as to the fact; but overlooking that for the 

 heat to be detected in the moonbeams requires the most 

 sensitive apparatus do not such observations assure us 

 that heat and light are independent of each other, phys- 

 ical principles having an existence separate from each 

 other ? 



Such were some of the arguments on which was sus- 

 tained the hypothesis of the intrinsic difference of light 

 and heat. In this, no account was taken of the optical 

 functions of the eye. Qualities were incorrectly attrib- 

 uted to radiations which, in truth, were due to peculiari- 

 ties in the organ of vision. 



The great service which the diffraction spectrum has 

 rendered to science is the abolishment of all these im- 

 aginary independent existences heat, light, actinism, 

 etc. and the substitution for them of the simpler con- 

 ception of vibratory motions in the ether. The only 

 difference existing among the radiations that issue from 



