Page:Philosophical Transactions of the Royal Society A - Volume 184.djvu/9

spheric thicknesses. Now, as the ray is known which, if observed, would give the same alteration in luminosity as that of the whole spectrum, it follows that with these data the equivalent ray for the photographic light can be deduced as shown in § XXV.

Fig. 4.

To ensure accuracy several sets of double cells, $\mathrm {C} _{1}\ \mathrm {C} _{2}$, were prepared, as shown in the figure (fig. 4). The top cell, $\mathrm {C} _{1}$, contained turbid water, the turbidity being caused by suspended mastic. The mastic was very cautiously precipitated and was thus held in suspension. As a matter of fact turbid water of different turbidities was prepared nearly two years before it was employed; and thus any coarse particles had ample time to settle; for the vessels containing it were kept undisturbed during that period and the liquid was syphoned off as required. The bottom cell, $\mathrm {C} _{2}$, contained pure water. Behind the cell the exposing box (fig. 1), already described, was placed, and in front of $\mathrm {C} _{2}$ sectors which rotated by means of clock work. The sectors could be clamped at any desired aperture. The apparatus, as shown, was then placed on a stand at such an angle that the suns rays fell directly on the platinum paper inside the box $\mathrm {B}$, when the exposing apertures were open. The sectors were closed to the extent which it was judged should suffice to make the blackness of the two exposed squares of platinum paper approximately the same, though it will be seen by the experiments to be subsequently described that this was not always attained. Exposures for different lengths of time were given in each experiment, and consequently any small error in measurement, or from any other cause, became insignificant when the mean was taken.

XXX.—Examples of the Method.

The following are examples showing the accord between the different values found for $\Lambda _{1}^{-4}$.

Experiment I.—Sunlight—length of cell (inside measures), $3\cdot 15$ inches; breadth