64 Royal Society :-— 
of a place, and points the way to relations for the chemical actions 
of the solar rays, which in the thermic actions are already repre- 
sented by isothermals, isotherals, &c. 
In order to determine the chemical action exerted by the whole 
diffuse daylight upon a given point on the earth’s surface, the 
authors were obliged to have recourse to an indirect method of ex- 
perimenting, owing to the impossibility of measuring the whole 
action directly, by means of the sensitive mixture of chlorine and 
hydrogen. For the purpose of obtaining the wished-for result, the 
chemical action proceeding from a portion of sky at the zenith, of 
known magnitude, was determined in absolute measure, and then, 
by means of a photometer, whose peculiar construction can only be 
understood by a long description, the relation between the visible 
illuminating power of the same portion of zenith sky and that of 
the total heavens was determined. As, in the case of lights from 
the same source but of different degrees of intensity, the chemical 
actions are proportional to the vdszb/e illuminating effects, it was 
only necessary, in order to obtain the chemical action produced by 
the total diffuse light, to multiply the chemical action of the zenith © 
portion of sky by the number representing the relation between the 
visible illumination of the total sky and that of the same zenith 
portion. 
The laws according to which the chemical rays are dispersed by 
the atmosphere can only be ascertained from experiments made 
when the sky is perfectly cloudless. In the determinations made 
with this specially-arranged photometer, care was therefore taken 
that the slightest trace of cloud or mist was absent, and the relation 
between the visible illuminating effect of a portion of sky at the 
zenith and that of the whole visible heavens, was determined for 
every half-hour from sunrise to sunset ; the observations being made 
at the summit of a hill near Heidelberg, from which the horizon was 
perfectly free. 
The amount of chemical illumination which a point on the earth’s 
surface receives from the whole heavens, depends on the height of 
the sun above the horizon and on the transparency of the atmo- 
sphere. If the atmospheric transparency undergoes much change 
when the sky is cloudless, a long series of experiments must be made 
before the true relations of atmospheric extinction of the chemical 
rays can be arrived at. The authors believe, however, founding 
their opinion on the statement of Seidel in his classical research on 
the luminosity of the fixed stars, that the alterations in the air’s 
transparency with a cloudless sky are very slight ; and they there- 
fore think themselves justified in considering the chemical illumi- 
nation of the earth’s surface, on cloudless days, to be represented 
simply as a function of the sun’s zenith distance. Although, from the 
comparatively small number of experiments which have been made, 
owing to the difficulty of securing perfectly cloudless weather, the 
constants contained in the formulz cannot lay claim to any very 
great degree of accuracy, the authors believe that the numbers ob- 
tained are sufficient to enable them to determine the relation accord- 
