] 50 Intelligence and Miscellaneous Articles. 



the largest was 72 ; the mean of the twelve was 32 centimetres. 

 .Starting from this, and from the mean value 31' of the angle sub- 

 tended by the moon's diameter, M. Thirion finds about 35 metres 

 for the distance sought — that is to say, for the mean of those to 

 which his pupils respectively referred our satellite. 



Lastly, Dr. Charpentier*, who wanted, for the discussion of an 

 entoptic phenomenon, to know to what distance his eye " projects 

 the retinal images in the vision of remote objects," determined 

 accessorily, by again a different procedure, that at which he places 

 the moon. That heavenly body appears to him to have a diameter 

 of only 15 centim., and to present the same breadth as a disk of 

 7*5 millim. diameter held at 645 millim. from the eye. From this 

 he deduces, for the apparent distance of the moon, the value 12-9 

 metres. 



From all this it follows, in the first place, that the distance in 

 question varies considerably from one observer to another, and, in 

 the second place, that it is always much less than would a priori 

 have been believed. 



I have applied my accidental -image process to the investigation 

 of an element for the approximate determination of which I believe 

 it is the only one practicable. When in the daytime and in calm 

 weather we look at the sky, it presents to us the appearance of an 

 azure surface in the form of an elliptical vault ; but at what dis- 

 tance do we picture to ourselves this imaginary surface ? Our eyes 

 then dive into an indefinite medium ; and yet they stop somewhere ; 

 but where? To what distance do they adapt themselves? It is 

 this, again, that the accidental images will permit us to appreciate. 

 As I said before, I reside in a house situated in a square, and with 

 its frontage facing the south. The experiment was made as fol- 

 lows : — In July last, about 4 p. M., the weather being calm, a square 

 of white paper marked with a black point in the centre was placed 

 upon a black surface : and the whole was put on the sill of an open 

 window. My son-in-law contemplated for 20 seconds the white 

 square, keeping his eyes fixed on the black point ; he then gazed at 

 the sky, at a sufficient height above the opposite houses of the 

 square, in order to observe the dark accidental image, after which 

 he projected this upon the wall of one of those houses, and found 

 that it had increased a little in magnitude : the respective sides of 

 the image in the two conditions were, in his estimation, in the 

 ratio of 5 to 6. My son Felix performed the same operations ; and 

 for him the ratio was about 4 to 5. The two observers were at 36 

 metres from the houses opposite ; consequently it follows, from 

 the proportionality above mentioned, that my son-in-law referred 

 the celestial vault to the distance of 30 metres approximately, and 

 my son to that of 29 metres. Therefore it is an equally inconsi- 

 derable distance that is attributed to the vault of heaven. 



* "Illumination violette de la ratine sous Finfluence d' oscillations lumi- 

 neuses," Comptes Rendus, t. xcii. 1881, p 355. 



