in their relation to the Seasons. 249 



According to M. Daguerre, the hours of the morning and of 

 night, equally removed from mid-day, and corresponding to 

 similar heights of the sun above the horizon, are not, however, 

 equally favourable for the production of photographic images. 

 Thus, in all the seasons of the year, and by atmospherical cir- 

 cumstances in appearance exactly' similar, the image is formed 

 a little more quickly at 7 in the morning, for example, than at 

 5 in the afternoon ; at 8 than at 4 ; at 9 than at 3. 



It is not from mere curiosity that we have noticed this ap- 

 proximation. The chemical action of light might well be 

 more foreign to these facts of the invasion and exacerbation 

 of certain diseases at different hours of the diurnal revolution. 

 The human body is more sensible than the most exact instru- 

 ments, and, above all, its sensibility is of a different descrip- 

 tion. We see, then, as was previously presumed, that the dis- 

 covery of M. Daguerre and his new reagents will receive 

 more than one scientific application. This discovery ^vill ex- 

 ercise its influence on many phenomena, on those even which 

 form the source of physiology and medicine. 



We would particularly recommend to M. Fuster this new 

 field of meditations ; it enters quite naturally into the series 

 of his researches, and no one is better fitted to improve it. 



The gi'eat fact of the diurnal movements does not present 

 itself only in certain acts of human life ; the exciting action 

 of light and heat is often manifested in a different, and some- 

 times even opposite manner, not only on individuals, but also on 

 species among animals and vegetables. We find nocturnal 

 species among the mammifera, birds, reptiles, mollusca, and 

 insects. 



AVe also meet with the phenomenon of dim-nal movements 

 in many functions of the vegetable life ; every one knows the 

 sleep of plants, the horologe of Flora, &c. Draparnaud has 

 observed, that at the end of autumn, when the temperatui-e 

 begins to fall, the flowers of the Iponnea violacea, and of the 

 Mirabilia, which, till then, are only open dming the night, 

 open at this epoch also during the day. 



The most exact observations have established these diurnal 

 movements in the different phenomena of meteorology. Such 

 are the diurnal motions of the compass and the barometer ; it 



