366 M. PONTON, ESQ., ON SOLAR LIGHT, 
The little box at the end of the sun-tube had now its cover put on, so as to 
leave open only its right-hand side, to which the lamp was approached, and dis- 
posed in the manner before described. The aperture applied to this tube was 
now 0:1, and, after trying several approximate apertures for the sky-tube, one 
of 0:0275 was ultimately fixed upon, as that which rendered the two images of 
exactly equal brightness. 
These observations were subsequently repeated, with the same results ; and, 
on each occasion, the apertures were, after the observations, examined under the 
microscope and ascertained to be clear. These results may therefore be regarded 
as a fair approximation to the truth. 
The brightness of the images being inversely as the areas of the apertures, 
it follows from the first observation, that a small surface illuminated by the 
direct rays of the sun at an altitude of 45° is 33°6 times brighter than a similar 
surface illuminated by diffuse day-light; while, from the second observation, it 
follows that a like surface illuminated by the flame of a moderator lamp, at the 
distance of 2 inches, and placed obliquely so that the rays might fall as nearly as 
possible at a horizontal angle of 45°, is 13-2 times less bright than a similar 
surface illuminated by diffuse day-light. Hence, the same surface when lighted 
by the sun is 444 times brighter than when lighted by the lamp, under the above 
circumstances, the blue rays only being used in each case. This exclusion of all 
but the blue rays, is somewhat adverse to the artificial light, which has an excess 
of red and yellow rays, beyond what is required for the composition of white 
light; but the blue rays may be held to indicate the proportion of white light, 
contained in the artificial flame. 
As, from the preliminary observation, it was found, that the moderator lamp 
employed was 3°5 times brighter than a wax candle (short 6 in the Ib.), it fol- 
lows, that a small surface, illuminated by mean sunshine, is 1554, or say 1560 
times brighter than is the same surface when lit by such a wax candle placed 
at 2 inches from it, in an oblique direction. 
Now, it is found not difficult to raise the electric light to such a pitch of in- 
tensity as to afford a light equal to that of 520 wax candles; so that, if the 
moderator lamp were replaced by three such electric lights, the surface would be 
equally bright as when illuminated by mean sunshine. 
To form a conception, therefore, of the quantity and intensity of the light 
emanating from the sun, when it reaches a distance of 95 millions of miles from 
his centre, we may imagine the surface of a sphere, having that distance for its 
radius, to be covered all over with a very thin film, say zgooth of an inch in thick- 
ness, having a brightness equal to that of an electric light of the above-mentioned 
intensity, and that behind this there are two similar films of equal brilliancy, 
the three forming a thin stratum, say goth of an inch in thickness; then such a 
stratum would represent the brilliancy of the sun’s light at the earth’s orbit. 
