NATURAL PHILOSOPHY. 147 



tensity is always represented by a straight line. This variation 

 in the direction of the straight line is due to the opalescenee of 

 the atmosphere ; and the authors show that, for equal altitudes, 

 the higher intensity is always found where the mean temperature 

 of the air is greater, as in summer, when observations at the 

 same place at different seasons are compared, or as the equator is 

 approached, when different places are examined. The differen- 

 ces in the observed actions for equal altitudes, which may 

 amount to more than 100 per cent, at different places, and to 

 nearly as much at the same place at different times of the year, 

 serve as exact measurements of the transparency of the atmos- 

 phere. 



The authors conclude by calling attention to the close agree- 

 ment between the- curve of daily intensity, obtained by the above- 

 mentioned method at Lisbon, and that calculated for Naples by a 

 totally different method. Philosophical Magazine, July, 1870. 



PHOTOGRAPHY AND MOLECULAR PHYSICS. 



Mr. Harrison, in "Nature," calls attention to the facilities of- 

 fered for studying molecular physics in connection with the 

 mechanical operations connected with photography. If 3 

 collodionized plates be dipped, one in a solution of oxide of cad- 

 mium, the second in a solution of bromide of cadmium, the third 

 in a solution of chloride of cadmium, as is done in " iodizing " the 

 plate, and each then sensatized by immersion in a bath of nitrate 

 of silver, the third will take longer than the second to become 

 covered with a film of silver salt, and the second longer than the 

 first. Also, the exposures to light in the camera when used for 

 taking a picture are in the same order. These differences in time 

 of exposure may, perhaps, be accounted for on the supposition 

 that the chlorine binds itself to silver with more force than is ex- 

 erted by bromine, and that the bromine clings to the silver with 

 more force than the iodine, so that the waves of light have more 

 work to do in heating the chlorine from the silver than in separat- 

 ing the bromine or the iodine. An experiment by Mr. M. Carey 

 Lea, of Philadelphia, shows that the atoms separated by light may 

 recombine in darkness. He prepared a film of dry iodide of 

 silver, on exposing which under a negative, a picture was formed, 

 which came out on developing with "alkaline developer." If, 

 however, instead of developing the picture, it was left in darkness 

 for a few days, the latent image died out, and a new picture could 

 be taken on the sensitive surface. 



MICROSCOPIC PHOTOGRAPHY. 



A very successful series of experiments on the production of 

 photo-micrographs, by means of artificial light, has been con- 

 ducted by Col. Woodward, of the Army Medical Museum, Wash- 



