Intelligence and Miscellaneous Articles. 149 



law of radiation at high, temperatures, and in regard to the mea- 

 surement of those high temperatures by photometry. I shall 

 shortly resume these two important questions, at present confining 

 myself to indicating the formula 



I=»iT 3 (l + ea- T ) T 



as well representing the results. I is the intensity of a simple 

 radiation; T, the absolute temperature; m, e, and a, constants 

 which will have to be determined. — Comptes Rendus de lAcademie 

 des Sciences, t. xcii p. 866 (1881). 



A3" APPLICATION OF ACCIDENTAL IMAGES. (SECOND NOTE.) 

 BY J. PLATEAU. 



In my first Xote* I showed that the distance to which we refer 

 the moon in the sky can be pretty exactly estimated by projecting 

 upon a wall the dark accidental image which succeeds the contem- 

 plation of the moon itself, then receding from or approaching 

 towards the wall till the diameter of the image is judged to be equal 

 to that of the celestial object, and, lastly, measuring the distance 

 at which the observer is from the wall when this condition appears 

 to be fulfilled. I said then that one of my sons had by this means 

 obtained about 51 metres for the distance in question. As this 

 value seemed very small, I have more recently induced my son to 

 make a control experiment, although with less precision : he sought 

 a position, in front of the wall, such that the diameter of the dark 

 image appeared to him as exactly as possible half of that of the 

 moon; and this required him to be distant from the wall not more 

 than 23-5 metres. Xow, in virtue of the known proportionality 

 between the diameter of the accidental image and the distance to 

 which it is projected, double the above quantity, viz. 47 metres, 

 consequently represented approximately the distance at which my 

 son instinctively placed the moon ; and it will be seen that this 

 latter distance, though not coinciding with the 51 metres of the 

 previous experiment less subject to error, is nevertheless of the 

 same order. 



M. Fabbe Thirion, Professor at the College de la Paix at Xamur, 

 has since published t an estimate at which he arrived by a quite dif- 

 ferent procedure. He assembled twelve of his pupils, and asked 

 each of them to trace on a black board a circumference of the size 

 he saw the moon in the skv. These twelve circumferences were 

 very unequal ; the smallest was only 19 centim. in diameter, while 



* Bull Acad, Roy. Rely. [2] xlix. 1880, p. 316; Phil. Mag. August 

 1880, p. 134. 



t La Lune — les prejuges et les illusions : Bruxelles, 1881. 

 Phil. Mag. S. 5. Vol. 13. No. 79. Feb. 1882. N 



