282 Dr. E. Lommel on the Theory of the 



disk like the moon in the horizon, I believe that the solid parti- 

 cles suspended in the atmosphere are adequate to explain this 

 coloration. A similar appearance is exhibited by the sun also 

 in the presence of a thick yellowish vapour (the Hohenrauch) , 

 even when it is high in the heavens. The red colour, too, which, 

 according to travellers' statements, the sun exhibits when the 

 simoom has raised the sand of the desert, belongs to this category. 



The spectrum of the setting sun has been lately more accu- 

 rately examined by Janssen. Towards the violet end it appears 

 continually weakened ; yet in the less refrangible parts, while 

 the intensity of light is otherwise unweakened, dark lines occur 

 along with the Fraunhofer's which are already there and have 

 become more distinct. This latter phenomenon is, in my view, 

 to be ascribed to the absorptive influence of aqueous vapour, 

 while the general enfeeblement of the more refrangible rays 

 depends upon the diffracting action of the finer particles of water 

 and of dust. 



By the same principles is explained the fact that imperfectly 

 transparent media (those, that is to say, which are turbid owing 

 to the admixture of very fine particles) transmit the less refran- 

 gible rays more easily than the others. Such media, for in- 

 stance, are liquids rendered turbid by finely divided precipitates, 

 smoky glass, milky glass, perhaps also dark opake glass (which 

 readily transmits the obscure thermal rays), smoke also, and a 

 glass plate blackened with soot. 



I also include here the observation first made by Hankel, that 

 light which is reflected from a ground glass plate under a very 

 oblique incidence appears of a reddish colour. If white light is 

 incident on the plate under a smaller angle of incidence, the small 

 depressions between the projections will act like small concave 

 mirrors turned in different directions, and will disperse the rays 

 of light accordingly. Hence an image of the source of light 

 cannot be formed, and, in spite of numerous but irregular re- 

 flexions, the diffuse reflected light will appear white. But if the 

 incident rays are greatly inclined to the plate, that light alone 

 reaches the eye which is reflected from the tops of the projections 

 that have by polish been brought into one plane, as is at once 

 seen from inspecting fig. 2. Hence an Fio> ^ 



image of the source of light, of a red- 

 dish shade, will be seen ; for the slender 

 pencils of light which are reflected from ^0^^^ 

 the tops of the projections will act just as 

 if they had passed through very small apertures. 



6. In the Fortschritte der Physik for 1861, in discussing the 

 paper mentioned at the outset, the objection is urged against my 

 idea that the red light of the setting sun is caused by the dif- 



