620 PROPESSOE BUNSEN AND DE. H. E. EOSCOE’S PHOTO-CHEmCAL EESEAECHES. 
photo-chemical phenomena of vegetation. That this influence must be of the greatest 
importance, is evident from the varying effects produced in other photo-chemical phe- 
nomena by differences in the solar light. We need only call to mind, in proof of this 
assertion, the fact, well known to all photographers, that the amount of light, deter- 
mined photometrically, gives no measure for the time in which a given photo-chemical 
effect is produced, and that the morning light is always preferred to the evening light 
for the preparation of pictures, even should tlie light in the latter case be more intense. 
