88 ON THE RELATION OF OPTICS TO PAINTING. 



in the atmosphere, which never entirely disappears* 

 If, in a transparent medium, there are fine transparent 

 particles of varying density and varying refrangibility, 

 in so far as they are struck by it, they deflect the 

 light passing through such a medium, partly by reflec- 

 tion and partly by refraction ; to use an optical expres- 

 sion, they scatter it in all directions. If the opaque 

 particles are sparsely distributed, so that a great part 

 of the light can pass through them without being 

 deflected, distant objects are seen in sharp, well-defined 

 outlines through such a medium, while at the same 

 time a portion of the light which is deflected is dis- 

 tributed in the transparent medium as an opaque halo. 

 Water rendered turbid by a few drops of milk shows 

 this dispersion of the light and cloudiness very distinctly. 

 The light in this case is deflected by the microscopic 

 globules of butter which are suspended in the milk. 



In the ordinary air of our rooms, this turbidity is 

 very apparent when the room is closed, and a ray of 

 sunlight is admitted through a narrow aperture. We 

 see then some of these solar particles, large enough to 

 be distinguished by the naked eye, while others form 

 a fine homogeneous turbidity. But even the latter 

 must consist mainly of suspended particles of organic 

 substances, for, according to an observation of Tyndall, 

 they can be burnt. If the flame of a spirit lamp is 

 placed directly below the path of these rays, the air 



