382 ANNUAL OF SCIENTIFIC DISCOVERY. 



until the shadow in the line of the candle became equally intense 

 with the shadow in the line of the sun. The direct light of the moon 

 was compared with the light of a candle in the same manner." 



"In two particulars, which may have exercised a considerable in- 

 fluence," says Mr. Bond, in his memoir presented to the American 

 Academy, " Wollaston's experiments seem to be less deserving of 

 confidence than those of Bouguer, as it does not appear that in the 

 former the extinction of light by the earth's atmosphere was allowed 

 for. 



"Another objectionable feature is the means employed for diminish- 

 ing the sunlight by admitting it only through a very small aperture, 

 while for the moon the full disc was compared. It is not clear that 

 these dissimilar conditions may not have occasioned some disturbance 

 in the results. Bouguer, by using the same aperture upon both 

 objects, has avoided this risk of error." 



Furthermore, in comparing the relative brightness of the sun and 

 moon, it is obviously of importance, says Mr. Bond, to vary as much 

 as possible the methods of investigation, in order to render them quite 

 independent of each other, so far as the nature of the subject will 

 permit. The three principal sources of difficulty encountered are, 

 the extreme intensity of sunlight ; the difference in color between 

 the light of the sun or moon and that of the ordinary kinds of artificial 

 illumination ; lastly, the want of a constant standard of brightness, to 

 which either object may be referred. The best intermediate standard, 

 as regards the quality of the light, would undoubtedly be some form 

 of the electric light, if its constancy could be maintained for a suf- 

 ficient interval ; all other artificial standards having greatly inferior 

 intensity, and a more or less decided red or yellow hue when con- 

 trasted with solar light, and even with moonlight, which was scarcely 

 to have been anticipated. 



The image of the Drummond light compared with the image of the 

 sun, both reflected from a silvered globe, has a strong golden-yellow 

 hue. The " Bengola" light answers much better. It shows, however, 

 a decided tinge of pink when brought side by side with the pure 

 white of the solar image. At night it is of an intense white, with a 

 bluish glare; but when contrasted with the moon's image, it exhibits 

 a striking similarity in quality of light, with only the very slightest 

 cast of pink at times, suspected of the same character with that no- 

 ticed in the experiments on the sun, but no trace of the blue can 

 then be detected. It seems, then, that, in point of color, the light of 

 the sun and that of the moon are very nearly similar. After various 

 trials, therefore, the Bengola light was fixed upon by Mr. Bond as 

 the illuminator for comparison, and the following plan was adopted 

 by him in his experiments : 



A glass globe, having a reflecting surface of silver, 10.16 inches in 

 diameter, was placed in the open air, exposed to the sunlight. The 

 brightness of the image of the sun formed at its virtual focus was 

 compared with a single Bengola light, by receiving the light of both 

 objects upon a small reflecting sphere, which was moved towards one 

 or the other until their images, seen in it side by side, were judged 

 to be equal ; the distances of the globes from each other and from the 

 Bengola were then measured, and the Observation repeated. 



