REVERSIBLE AND IRREVERSIBLE SYSTEMS UNDER INFLUENCE OF LIGHT. 343 
A spectroscopic investigation of the light passing through the different screens 
showed that only the red light was perfectly pure, while the light passing through 
the blue and green-yellow screens contained very much red, the thickness of the 
gelatine having been far insufficient for its total absorption. This explains the above 
data with the thermopile. A pure red light, comparable with the total whole light 
was, however, all that was required for the present purpose. 
Table A, p. 344, gives the E.M F.’s obtained in the course of this research for 
different combinations with the total light and with the same light passing first 
through the red screen in each system expressed in the same units (millims.) taken 
from the photographed curves, one of the plates having been exposed to light, the 
other kept in the dark (under identical experimental conditions). 
The table shows that while the heating effect of the light passing the red 
screen upon the thermopile (Column VI.) is still 73’2 per cent, of the heating effect of 
the total light, the deflections caused in the different systems by the total light 
passing the red screen are only a fraction, mostly a small fraction (Columns IV. and 
V.) of the deflections caused by the total light without screen (Column III.). 
Thus the total deflection caused by the total light could not be due to the heating 
effect of light alone. In Column VII., the exact values of the thermo E.M.F.’s 
caused by the total light are given, as they follow from direct measurements of the 
rise of temperature at the illuminated front plate, and of the thermo E.M.F.’s per 
1° C. of the given system. Column VIII. gives the same values for the total light 
passing first the red gelatine screen. The difference between Columns III. (100 per 
cent.) and VII. gives the deflections caused by the total light at a constant tempera¬ 
ture in per cent, of the total deflection observed in Column III. This difference 
shows that almost the total deflection is under our conditions of experiment created 
by light at a constant temperature, and only a small part principally in systems (1), 
(3), and (5) is due to a thermo E.M.F. created by light. Column X. gives the 
difference between Columns V. and V III., i.e., the deflection at a constant temperature 
caused by the total light passing the red gelatine screen. This Column X. shows 
that in both cases of the systems (2) and in (3) the light passing the red screen gives 
pretty considerable deflections at a constant temperature, i.e., the red light is also 
“actinic.” In case of the other systems, e.g. (5) or (7), the total deflection caused by 
red light is very small, and it is difficult to show experimentally how much of this 
value is a thermo E.M.F., and how much an E.M.F. at a constant temperature. 
fable B, p. 345, gives the relative deflections observed for each of the systems 
given in the previous table with the total light, and the same light passing through 
the red, yellow, green and blue screens; the same values at a constant temperature 
are given in the last three columns. 
