PEOrESSOE BUNSEN AND DE. H. E. EOSCOE’S PHOTO-CHEMICAL EESEAECHES. 887 
we conclude tliat, in the case of different luminous sources, the amount of light received 
by the eye in no degree serves as a measure of the chemical action which the light can 
effect. 
3. Chemical Brightness.---lLhQ amount of light measured photo-chemically which falls 
perpendicularly from a luminous surface upon a physical point, divided by the apparent 
magnitude of the surface, we designate as the intrinsic chemical brightness. As unit of 
apparent magnitude, we select the thousandth part of a hemisphere ; and as unit of 
intrinsic brightness, we select the amount of light which must proceed from this thou- 
sandth part in order to communicate an illumination of 1 degree of light to a physical 
point placed in the centre of the hemisphere. 
In order to measure the amount of the chemical brightness of a surface, all we need 
is to allow the light proceeding from this sm'face to fall through a circular opening upon 
the insolation-vessel, and to measiue in degrees of light the chemical illumination thus 
effected. Let I signify the observed number of light-degrees, d the diameter of the 
circular opening, r its distance from the insolation-vessel; we then have 77^ as the 
luOO 
thousandth part of a hemisphere whose radius is r, and ^rr^ .2 bitC as the portion of 
this hemisphere occupied by the circular opening, when the value of 6 is obtained from 
the equation sin The portion of the hemisphere cut out by the circular opening. 
is to the thousandth part of the hemisphere in the proportion of 2 siiri ^ to 77777. Hence 
2 lUuO 
we obtain the intrinsic brightness H expressed in the above units from the following 
equation, — ; 
(3.) 
2000 sin^ - 
in which I signifies the number of observed degrees of light. 
As an example of such a measurement, we select a comparison of various sized circular 
portions of the zenith of a cloudless sky. The elements for this calculation are obtained 
fr’om the following experiment; — Outside the window of our dark room («a, fig. 3) 
was placed a mfrror (Z>), inclined at an angle of 45° towards the horizon ; by means of 
this mirror the light from the zenith could be reflected through a horizontal tube 
(c, fig. 3, Plate XLIII.) upon the insolation-vessel (^) in the dark room. On the end of 
the tube nearest the mirror, and outside the window, circular diaphragms were placed, 
the diameters {d) of which are given in column I. of Table III. The distance of the 
insolation-vessel from this opening was, as is seen in column II., for all the experiments 
2'“'225. Before the insolation-vessel, a screen (^, fig. 3, Plate XLIII.) containing two 
plates of mica was placed. The chemical action (w), effected in one minute, obtained 
from the mean of six observations made with the scale No. 2, is found in column III. 
The number of degrees of light {1) seen in column IV., which illuminate the insolation- 
vessel, is found from the equation N.L. E.?^.w=^, in which the following constant 
values occur ; — 
6 A 2 
