1916.] NATURAL SCIENCES OF PHILADELPHIA. 477 



carbon spectra in some cases. The luminosity of the tail, which, 

 it must be borne in mind is extremely low, according to any way of 

 rating it, is. I think, amply accounted for by the greal depth of >i>ace 

 in which the fine matter exists and the solar light of high intensity 

 reaching this fine matter: a pure case of diffusion from fine particles. 

 As to cosmic dust not reflecting sunlight as does fine cometary dust, 

 the following explanation, it seems to me, is ample. We sec things 

 by contrast. Even if there was a slight luminosity in space, it would 

 -imply form a background for the comet's tail, which would be more 

 luminous and more dense in dust. The astronomers have long sus- 

 pected a very thin veil of dust in the sky, of dust under illumination* 

 The sky is not, in fact, absolutely black. It is black by comparison. 

 Of course, it cannot be expected that ordinary meteorites should 

 ever be seen in the sunlight. They are too small or too far apart. 

 Only when they reach a considerable size do they produce any 

 impression as single spots of light. Witness the asteroids or the 

 little planet Eros, which probably is miles in diameter and yet is 

 difficult to detect. 



My idea is that the continual readjustment of the position and 

 relation of the parts within a small cluster, or even within a large 

 cluster forming a large comet, will account for practically all that 

 occurs in the head of the comet and in the tail. 



This tail will always point away from the sun. It will curve 

 backwards, for although the particles move in a straight line from 

 the sun, it will curve backwards owing to the progress of the nucleus 

 around the sun. The tail will continue to be formed so long as the 

 disturbing actions occur, and it will fade away as the comet gets so 

 far from the sun as not to be distorted or disturbed. If the comet 

 is not otherwise lost, this action will continue on from the action of 

 the sun with a constant reduction of mass and the final diffusion of 

 the material composing the comet. 



Your statement in relation to the aerolite of a stony nature 

 which you found is very interesting to me, as I had not heard 

 that you had found any such stony meteor. It is quite easy, how- 

 ever, it seems to me, to account for it as a survival of perhaps 

 many others present in the cluster. The stony meteorites will 

 naturally be sifted out from the iron meteorites in the flight of a 

 meteor through our atmosphere. They would not only be crushed 

 by air pressure on their relatively greater area per unit of mass, but 

 on account of their lower specific gravity they would not have the 

 ability by momentum to force their way as far. Their relatively 



