PEOPESSOE BinsrSEN AND DE. H. E. EOSCOE’S PHOTO-CHEMICAL EESEAECHES. 921 
instead of 2440 millims., the action, instead of being 0-01817 degree of light, would 
have been 
0-01817x24402 
208-72 
2-482 degrees of light. 
This distance (208-7) is, however, that at which the luminous circular area of burning 
magnesium seen from the insolation-vessel has the apparent magnitude of the sun’s disk. 
As the sun, before its rays suffered diminution by passing through the atmosphere, would 
effect upon the insolation-vessel an illumination of 318-3 degrees of light (seen from 
formula 14.), the chemical brightness, or the chemical activity of the rays proceeding 
from the sun s surface is or 128-2 times larger than that of the rays evolved from 
magnesium wire bm-nt in the way described. By help of the same formula (14.) it is 
seen that the chemical brightness of our incandescent magnesium wire is equal to that 
of the sun when it stands about 9° 53' above the horizon. A burning surface of mag- 
nesium wire, which, seen from a point at the sea’s level, has an apparent magnitude 
equal to that of the sun, effects on that point the same chemical action as the sun 
would do when shining from a cloudless sky at a height of 9° 53' above the horizon. 
If, for instance, such a surface had a diameter of 1 metre, the chemical action which it 
would produce at a distance of 107 metres is the same as the sun would effect when 
shining perpendicularly on any object from an elevation of 9° 53' above the horizon. 
As a matter of interest we have, in contradistinction to the chemical, compared the 
visual brightness of these two sources of light, i. e. the brightness as measured by the eye. 
For this purpose the direct rays of the sun at 12 o’clock at noon on the 13th of 
Nro\ ember, 1858, when the sky was cloudless, were reflected by means of a mirror of 
black glass through a circular opening of 0-399 millim. diameter on to the stearinized 
diaphragm of the photometer, and then the gas-flame of the photometer was so arranged 
that the diaphragm ring disappeared. In order to counterbalance the different colour 
of gas- and sun-light, a piece of pale blue glass was placed between the flame and the 
diaphragm. 
Let us suppose that the intensity of the light when the ring disappears is I. Let S 
be the intensity which the unit amount of sunlight possesses after the one reflexion, g 
the apparent area which the hole through which the sun passes appears to have when 
seen from the diaphragm, and the apparent area of the sun’s disk ; then the following 
fraction gives the intensity of the sun’s direct rays falling perpendicularly upon the 
diaphragm, 
S^’ 
In order to compare with this, the intensity of the burning magnesium, the wire was 
burnt before the diaphragm of the photometer, at such a distance that the ring, as in 
the case of illumination Avith the sun, disappeared. If we call g.^ the apparent area of 
the incandescent surface of the wire as seen from the photometer ring, the burning 
