184 On the Diatfiermancy of the Media of the Eye. 



a later hour of the day, when the sun's rays have traversed a 

 thicker layer of fog-vesicles than at midday. The ratio between 

 the quantities of heat belonging to the first obscure zone which 

 are incident upon the layer of water, and those traversing it, 

 must approach unity from morning to evening. 



But experiments by Melloni* and by Volpicellit contradict 

 this conclusion. Both physicists have found that a layer of water 

 enclosed between two glass plates diminishes the intensity of the 

 transmitted rays from morning to evening. They used the 

 unrefracted heat reflected by a heliostat. In order to extend 

 these experiments to refracted thermal rays I used a rock-salt 

 prism, and also a vessel of rock salt with parallel sides and filled 

 with concentrated solution of rock salt. The inside distance of 

 the sides of the vessel was 6 millims. The rays of the bright 

 zones at midday and at four o'clock traversed this layer of water 

 in the same manner ; and even for the rays of the dark zones 

 the differences were within the limits of the errors of observation. 

 The decrease in the intensity of the transmitted rays observed 

 by Melloni and Volpicelli cannot be recognized in these experi- 

 ments, perhaps partly because the differences in the thickness of 

 the layer of atmospheric air were not so great as in the experi- 

 ments of the above physicists. However, observations made in 

 the clear atmosphere of Italy are not to be compared, without 

 further consideration, with those made in our latitudes : thus Mel- 

 loni found the maximum heat in the spectrum of a rock-salt prism 

 so distant from the red end, that the distance between it and the 

 red was as great as the distance between the red and the violet J, 

 while in my observations the maximum never exceeded the first 

 dark zone. 



If, then, the question, what action the particles of water in the 

 atmosphere exercise upon the thermal rays traversing it, remains 

 undecided, it is not to be doubted that small solid particles floating 

 in the atmosphere exercise a great influence upon the rays which 

 traverse the whole atmosphere. A thin cloud of smoke, which 

 scarcely diminished the brilliancy of the solar rays, was traversed 

 by the sun's rays before impinging on the heliostat mirror, upon 

 which no heat could be perceived in the dark zones of the 

 spectrum. 



From these experiments, it follows that it is not possible to 

 obtain reliable quantitatively comparable results by using as 

 source of heat the sun in our latitudes, and especially in an 



* Poggendorff ' s Annalen, vol. lxxxvi. p. 496. 



f Cornptes Rendus, vol. xxxv. p. 953. 



+ PoggendorfPs Annalen, vol. xxviii. p. 377. 



