A. NEW PHOTOMETER. 207 



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 sensitive surface exposed to such 

 a eleam is more and more affected as the time is increased. 



O 



944. Considerations like these demonstrate the necessity of investigating the ques- 

 tion 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.) 



945. This doctrine, however, will not, I am persuaded, stand the test of criticism : 

 there are facts, and these very imposing ones, which make it utterly untenable. It is 

 my object, in this paper, to set forth that evidence, and offer farther and clearer proof 

 of the physical independence of the tithonic rays and light, and indirectly establish the 

 existence of a new imponderable. 



946. The true issue of the question, as has been said, rests in proving a clear dis- 

 tinction between light and the tithonic rays ; the other imponderables may be left out 

 of the argument. The mechanical properties of the two agents are so closely alike 

 reflexion, refraction, polarization, interference, &c. taking place under the same laws 

 for both, that the discussion necessarily becomes one of quantity and measure. Will 

 a given ray of light, disturbed by the action of absorptive media, change its luminous 

 and chemical relations pari passu ? or can we alter the one and leave the other un- 

 touched I Or, changing both by any process of treatment, do both change to the same 

 extent ? 



947. The final decision of this question obviously rests in obtaining accurate meas- 

 ures for the rays of light and for the tithonic rays. It is the comparison of those meas- 

 ures which is to settle the point. 



948. In Chapter XVI. I have described an instrument, under the name of the titho- 

 nometer, which gives indications by the production of muriatic acid from the union 

 of chlorine and hydrogen. This instrument is affected chiefly by the indigo rays, or, 

 more correctly speaking, by those rays which extend over the blue, indigo, and violet 

 spaces of the spectrum, having their maximum in the indigo. It is important that the 

 reader should keep this fact in mind. 



949. Optical writers have been greatly embarrassed for want of a photometrical in- 

 strument which can measure the intensity of light ; the chief difficulty in the way is the 

 impossibility of contrasting together lights that differ in colour. By all, it is admitted 

 that the eye is able to judge of the amount of illumination of white surfaces, or the 

 depth of shadows within small limits of error, provided the rays compared are nearly 

 of the same tint. 



950. But in the discussion on which I am now entering, this very difficulty is in- 



