Page:Philosophical Transactions of the Royal Society A - Volume 184.djvu/3

therefore, from such observations $\kappa$ could be at once determined. It was similarly shown that when a photographic silver salt was employed to register the photographic spectrum it was equivalent to measuring the integrated spectrum with a coefficient of absorption $\mu ^{\prime }$, and that this was equivalent to observing the alteration in intensity of a ray $\lambda \;4540$.

XXV.—Application to the foregoing Results.

This being the case, if by any photographic means we can measure the total intensity of white light affecting a photographic compound after passing through various air thicknesses, we ought to be able to find the value of $\mu _{1}$ in the formula $\epsilon ^{-\mu _{1}z}$,and if we also observe optically the value of light transmitted through various air thicknesses we shall get the value of $\mu$ in the formula $\epsilon ^{-\mu z}$. As already stated we know that $\mu =105\kappa$. Similarly, $\mu ^{\prime }=\Lambda ,^{-4}.\kappa$, $\Lambda$, being the equivalent ray observed.

Therefore

${\frac {\mu ^{\prime }}{\mu }}={\frac {\Lambda ,^{-4}}{105}}\quad$ or $\quad \Lambda ,^{-4}={\frac {105\mu ^{\prime }}{\mu }}\quad$ and $\quad \kappa ={\frac {\mu ^{\prime }}{\Lambda ,^{-4}}}$,

that is, we can find the single ray which is the equivalent of the whole of the spectrum which is impressed on any photographic compound, and having thus found $\Lambda ,^{-4}$ we can at once deduce the value of $\kappa$—or the coefficient of scattering by the fine particles.

XXVI.—Sensitive Compound employed.

The question which presented itself was as to the best form of sensitive salt to use for convenience and accuracy. A process with a silver salt requiring development was almost impracticable. The exposure necessary to give to sunlight would have been so small that accuracy in timing it would have become a very difficult problem. The chloride of silver paper which darkens by sunlight was experimented with, but it had a very serious drawback. Roscoe and Bunsen have shown that chloride of silver paper may be prepared, which, when exposed to the same intensity of light for the same time, will always give the same blackness, but it is impossible to keep this paper for more than a few hours, and it would often be inconvenient to prepare it. Further it can never be predicted when a day will be suitable for making observations, and measurements of its blackness would be difficult except at a fixed observing station. There were also two other desiderata which had to be taken into account; one was that the record of the action of light should be as permanent as possible, and the other that it should be easy to measure the action produced by the light. Chloride of silver paper fulfilled neither of these desiderata, since fixing the chloride altered all measurement and the darkening was liable to fade even when the paper was fixed, and it was not easy to obtain accurate measures of the darkening owing to its ruddy colour.

In the comparatively new process of platinotype we have, however, a process which