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 336 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 lb.), 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 looo^h of an mc h m 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 40th of an inch in thickness ; then such a 

 stratum would represent the brilliancy of the sun's light at the earth's orbit. 



