Curiosities of Science. 61 



GREAT SIZE OF THE SUN ON THE HORIZON EXPLAINED. 



The dilated size (generally) of the Sun or Moon, when seen 

 near the horizon, beyond what they appear to have when high 

 up in the sky, has nothing to do with refraction. It is an illu- 

 sion of the judgment, arising from the terrestrial objects inter- 

 posed, or placed in close comparison with them. In that situ- 

 ation we view and judge of them as we do of terrestrial objects 

 in detail, and with an acquired attention to parts. Aloft we 

 have no association to guide us, and their insulation in the 

 expanse of the sky leads us rather to undervalue than to over- 

 rate their apparent magnitudes. Actual measurement with a 

 proper instrument corrects our error, without, however, dis- 

 pelling our illusion. By this we learn that the sun, when just 

 on the horizon, subtends at our eyes almost exactly the same, 

 and the moon a materially less, angle than when seen at a 

 greater altitude in the sky, owing to its greater distance from 

 us in the former situation as compared with the latter. Sir 

 John HerscheVs Outlines. 



TRANSLATORY MOTION OF THE SUN. 



This phenomenon is the progressive motion of the centre 

 of gravity of the whole solar system in universal space. Its 

 velocity, according to Bessel, is probably four millions of miles 

 daily, in a relative velocity to that of 61 Cygni of at least 

 3,336,000 miles, or more than double the velocity of the revolu- 

 tion of the earth in her orbit round the sun. This change of the 

 entire solar system would remain unknown to us, if the ad- 

 mirable exactness of our astronomical instruments of measure- 

 ment, and the advancement recently made in the art of ob- 

 serving, did not cause our progress towards remote stars to be 

 perceptible, like an approximation to the objects of a distant 

 shore in apparent motion. The proper motion of the star 61 

 Cygni, for instance, is so considerable, that it has amounted 

 to a whole degree in the course of 700 years. Huntboldt's Cos- 

 mos, vol. i. 



THE SUN'S LIGHT COMPARED WITH TERRESTRIAL LIGHTS. 



Mr. Ponton has by means of a simple monochromatic pho- 

 tometer ascertained that a small surface, illuminated by mean 

 solar light, is 444 times brighter than when it is illuminated by 

 a moderator lamp, and 1560 times brighter than when it is 

 illuminated by a wax-candle (short six in the Ib.) the artifi- 

 cial light being in both instances placed at two inches' distance 

 from the illuminated surface. And three electric lights, each 



