104 Dr. Draper's Further considerations 



veloped perhaps in a less elaborate way, but under the gui- 

 dance of a principle equally ajthereal and refined. The beams 

 of the stin are the true nervous principle of plants. To the 

 yellow ray is assigned their nutritive processes, to the blue 

 their movements. We can therefore easily understand how 

 it is that botanists who have sought in the interior of plants 

 for indications of a nervous agent have never found them. 

 That agent is external. 



By the experiments that have been made for determining 

 the nature of the chemical radiations, the question has been 

 brought down to very narrow limits. There is no author who 

 regards them as connected in any way with radiant heat, nor 

 any, except the wildest speculator, who traces them to elec- 

 tricity. The difficulty is to offer clear and undoubted proof 

 that they are distinct from light. Herschel has directly ad- 

 mitted this distinction and brought forward several experi- 

 ments (Phil. Trans. 1840, p. 38, &c.) in support of this view. 

 I have given some evidence of the kind, both recently and 

 also several years ago (1837). All these experiments de- 

 pend on a comparison of tithonographic stains produced by 

 solar spectra that have undergone the action of absorptive 

 media, and the effect of those spectra on the organ of vision ; 

 a comparison, in short, of different visible spectra and their 

 tithonographic impressions. 



From this comparison we endeavour to prove that invisible 

 rays may be isolated in any part of the spectrum, and, if invi- 

 sible, we argue that they are not light. 



It cannot be concealed, however, that there is a certain 

 degree of imperfection in this species of evidence : an accurate 

 conclusion as to the presence or absence, or quantity of light, 

 is by no means under these circumstances an easy affair. In 

 these distorted spectra, as in the natural one, the terminations 

 shade off gradually, and it is difficult to say where the light in 

 reality ends. On these terminations also, where the light is 

 so dilute and feeble, tithonographic action, although faint, 

 may, by prolonged exposure, become not only perceptible but 

 even prominent. In tithonographic action time enters as an 

 element, in the act of vision it does not. A feeble gleam does 

 not become more bright by constantly looking at it, but a sen- 

 sitive surface exposed to such a gleam is more and more af- 

 fected as the time is increased. 



Considerations like these demonstrate the necessity of inves- 

 tigating the question in other ways, and more especially since 

 M. Becquerel, one of the ablest writers on these matters, has 

 undertaken to support a doctrine which denies the existence 

 of the chemical rays and imputes the whole action to light 

 (Taylor's Scientific Memoirs, vol. iii. part 12). 



