280 Dr. E. Lommel on the Theory of the 



thus to produce, along with the enfeebled direct white ligh t, red- 

 dish-coloured oblique incident light. If these, before reaching 

 the eye, again fall on a similar group of small screens, all (the 

 direct as well as the diffracted) will again undergo the diffract- 

 ing action. The direct rays in passing through the second group 

 will diminish in intensity but not in whiteness ; they will more- 

 over give rise anew to the production of reddish diffracted light. 

 The diffracted rays are again diffracted by the second group ; of 

 each diffracted pencil of rays one part is bent back in the direc- 

 tion of the direct rays. This had already suffered a loss of more 

 refrangible rays by its first diffraction ; on the second diffrac- 

 tion the more refrangible rays are again weakened to a greater 

 extent than the less refrangible, its tendency to a reddish colour 

 will therefore increase. With the direct light, now become weaker 

 as compared with what it previously was, reddish light diffracted 

 in the same direction will be associated, and thus make its tint 

 reddish, though perhaps almost imperceptibly so. It is easily 

 seen that similar reasoning applies to the diffracted pencil of 

 rays which goes direct through the second group. Hence there 

 is added to the light which is red from the first diffraction that 

 which by the second diffraction has become parallel with it and 

 is of a still deeper red. 



By the addition of further groups of small screens parallel 

 with the first, the original white direct light becomes more and 

 more weakened, while there is continually being mixed with 

 it more light which by repeated diffractions has become of a 

 deeper red. The successive groups of screens act, so to speak, 

 like sieves, which continually separate the transmitted light 

 more and more perfectly from its more refrangible rays. Hence 

 the white point, viewed through a sufficient number of such groups 

 of screens, not merely appears reddish of itself, but also appears 

 surrounded by a still more strongly red- coloured aureole of dif- 

 fracted light. 



If the rays, instead of being condensed by a lens, are received 

 on a white screen, this, as is obvious, must appear as if illumi- 

 nated with red light. 



For the production of phenomena of diffraction it is unnecessa. / 

 that the screens be perfectly opake ; it is enough that less light 

 passes through them than by them. Thus the phenomena of 

 halos, already mentioned, is produced by the diffracting action 

 of fog-vesicles, which are themselves transparent. 



5. The explanation of the red colour of the sun in rising and 

 setting is now evident from what has been said. In the lower 

 layers of the atmosphere a number of fine corpuscles are sus- 

 pended ; it is immaterial whether they be solid (as, for instance, 

 organic and inorganic dust, the fine particles of carbon in smoke) 



