36 



Jupiter, and 80deg, for the Moon. Their relative distances 

 from the Sun were : — 



As 1-0155 to 5-4562 



Theoretical illumination (as squares) 1 to 28-8 



As measured 4 to 9, or 1 to 225 



Actual exceeds theoretical ... &|^_ — 12-7 times. 



I had not yet done with the question. In my experi- 

 ments I had, by various means, stopped back all but the 

 smallest fraction of the light received. The inquiry arose, 

 did this small amount represent equal percentages from both 

 bodies ? I settled this question at home thus : — A small 

 hole iV inch in diameter was pierced in a bit of sheet metal, 

 and covered with tracing paper. This, with a flame behind 

 it, made a capital artificial planet. I improvised a small 

 telescope out of a short focus photo, combination, using the 

 same eye-pieces as before, and the dark wedge. To this 

 arrangement I adapted a graduated shutier as before des- 

 cribed. The light — a small gas flame — was enclosed in a 

 magic lantern, the lenses being removed, Measures were then 

 taken of the intensity of light as received upon the spot of 

 tracing paper, the flame being at 1, 2, 3, 4, and 5 feet distant 

 respectively. The average of a series of readings for these 

 several distances in order was 180, 110, 61, 31 and 6 degrees. 



Theoretically they should have been 175, 112, 63, 28 and 7 

 degrees. 



This was, I think, sufficiently near to show the general 

 correctness of the principle. 



To sum up then — as compared with Mars, I make Jupiter's 

 surface brightness to be 22 times as great as it ought to be — 

 and as compared with the Moon 12-7 times. 



These comparisons, of course, go on the assumption that 

 the surfaces to be compared are equally reflective, that is, of 

 equal whiteness. That they are really not so may bo con- 

 sidered as certain. But even supposing Jupiter's surface to 

 be of the whiteness of mow, -we have at the polos of Mars 

 planetary snow with which to compare it, and illuminated 

 with 12 times the intensity of Jupiter's sunshine ; yet, even 

 this comparison, I judge to bo in favour of Jupiter. As 

 compared with the Moon, the vaporous envelope of Jupiter 

 (whatever its nature) is probably more reflective than the 

 bare and broken lunar surface. There must, however, remain 

 a vast amount of illumination to be accounted for in some 

 other way. 



I must say that the result of my measures appears 

 incredible even to myself. I, therefore, look for some 

 corroboration. According to Mr. G. P. Bond's estimate, 

 the light we receive from Jupiter amounts (at a mean) to w ^ v 

 of that of Ml moonlight. Jupiter's visual disc (mean) is ^^ 



