494 



Prof. C. V. Boys. 



[Apr. 24, 



time. It was not moved, nor was anything moved, except the shutter 

 in the beehive. At each movement of the shntter the image of the 

 light started off violently, and came to rest in about five seconds, 

 showing a deflection of 38 mm. This was repeated about twenty 

 times. This is an absolutely trustworthy measure of the sensibility 

 of the whole apparatus, including imperfect reflection by the silver 

 and every factor, except the absorption by the air. This, as the night 

 was dry and clear, must have been very small. It shows if \ mm. 

 is taken as the smallest deflection which can be observed with 

 certainty, that a candle-flame at 250*7 yards produces an effect which 

 is 152 times as great as the least which can be certainly observed • or 

 that, if the candle-flame had been taken a/152 or 12*3 times as far away, 

 that is, 3084 yards, or 1*71 miles, it would have sent into the telescope 

 sufficient heat to be observed, and therefore more than Arcturns. 



After these measures were made, the stand, &c, were removed, and 

 my niece stood with her face exactly in the line of the telescope. 

 The image of her face must have been entirely within the disc. 

 When she moved on one side the clear sky was in view instead. The 

 deflection, observed many times, was 48 mm. This large effect was no 

 doubt chiefly due to the screening of the sky, but I mention the 

 experiment to show that the mirrors reflect dark heat satisfactorily. 



Later on in the evening, at about 10 p.m., I turned the telescope on 

 to the moon, now about full. The index was, of course, driven right 

 off the scale, but with a 6-inch stop over the mouth, the deflection, 

 due to the centre of the moon's disc, was 150 mm. With a 12-inch 

 stop the index again went off the scale. Now the area of the mirror, 

 which is 15*625 inches in diameter, is 192 square inches, the area of 

 the 6-inch stop is 28*25 square inches, and the area obstructed by the 

 flat is 4*67 square inches. The effective area of the mirror is there- 

 fore 187*3 square inches, and of the 6-inch aperture 23*58 square 

 inches, and the ratio of these numbers, 7*95, is the number by which 

 the deflection with the 6-inch stop must be multiplied in order to 

 obtain the deflection that would be obtained from the whole mirror if 

 all could still go in proportion. The centre of the full moon, then, 

 would give, on this supposition, a deflection of 1192*5 mm., or about 

 4800 times as great an effect as could be observed with certainty. 

 Since the disc is in the particular instance about one-sixth of the 

 diameter of the image of the moon, it is clear that it covers about 

 1/36 or receives, in round numbers, 1/30 of the total heat, if it is at 

 the centre. It is evident, then, that an amount of heat equal to 

 about 1/150,000 of that sent by the full moon would produce an effect 

 which could be observed with certainty, and Arcturus does not send 

 this. 



Dr. Huggins Jid not obtain any effect from the moon, but, as he 

 used an object glass which would absorb nearly all the moon's heat, 



