the Media of the Eye. 183 



The deflections in the red zone amounted to about thirty-five 

 divisions of the scale. It will thus be seen that apparently- 

 bright days, like the 12th of August, sometimes cannot be used 

 for experiments with obscure solar heat. On two days only 

 have I obtained such an absorption of the obscure rays, that the 

 absorption in the first obscure zone was greater than in the red. 

 Hence, before each series of experiments like the above, the 

 solar heat which reaches the surface of the earth without any 

 absorbing obstacle had previously to be tested. In this preli- 

 minary trial, a phenomenon was observed which rendered a com- 

 parison of the results more difficult : the heat in the first obscure 

 zone decreased with the sinking of the sun as compared with 

 the heat in the red, although no difference was perceptible in 

 the external appearance of the sky. In the first obscure zone, 

 on a very bright day, experiment gave the following results for 

 times of observation about an hour apart : — 



13-00, 12-26, 11-78, 11-60, 



if the quantity of heat contained in the red zone (which was 

 determined anew before each observation) is indicated by 10*00. 

 Similar results were obtained on other days. Hence, on the same 

 day, and under otherwise similar circumstances, the loss of obscure 

 thermal rays accompanying luminous rays increases with the 

 thickness of the layer of our atmosphere which the sun's heat 

 traverses. The general conclusion does not, however, follow, 

 that the quantity of obscure rays always decreases with the thick- 

 ness of the layer of atmospheric air. For the ratio of the quanti- 

 ties of heat in the first obscure zone and in the red, the mean of 

 all the series of observations in a year was, 



In July 10-64 : 10-00 



In the first half of August . . . 1074 : lO'OO 



In the second half of August . . 12-87 : 1000 



In September 13-59:10-00 



The increasing thickness of the layer traversed is not therefore 

 alone the reason of the above observed decrease of the obscure rays; 

 it is to accidental constituents of the atmosphere, fog-vesicles, or 

 finely-divided solid substances that this disturbing influence is 

 due, either by absorption or by reflexion. 



We know, from Melloni's investigation and those of other 

 physicists, that the heat which has traversed a layer o/ a dia- 

 thermanous substance penetrates a second layer of the same sub- 

 stance the more easily in proportion as the first layer was thicker. 

 Hence if fog-vesicles floating in the air were the cause of the 

 variation in the absorptive power of the atmosphere, less obscure 

 heat ought to be absorbed at midday by a layer of water than at 



