yello'do Raj/s on Dagnerreotijpe Plates. 205 



liglits reflected from the atmosphere explains those anomalies 

 which are so annoying to photographers; for example, those 

 circumstances uniier which it seems impossible to obtain any 

 image, whatever may be the length of exposure in the camera. 

 This difficulty is imputed to the preparation of the plates, the 

 state of the accelerating solutions, &c. ; and alter all it only 

 exists in the light. As much effect is often obtained in 30 

 seconds as in 120 seconds, for it is possible to have obtained a 

 first effect which has been afterwards destroyed ; and that must 

 inevitably be the case if, during the latter part of the exposure 

 in the camera obscura, any clouds or vapours have suddenly 

 rendered the light of the sun yellow; in this case the object 

 which had first reflected white light, becoming capable of re- 

 flecting only yellow light, this last must destroy the effect pro- 

 duced by the first. 



If the object reflects at the same time blue light from the 

 zenith and yellow light from the southern horizon, the two 

 lights may be neutralized and destroy each other, so that no 

 photogenic effect can be obtained. Sometimes, when the sky is 

 cloudless, the space at the zenith is of a particularly dee]) blue 

 colour; whereas the vapours produced by the heat of the sun 

 make the sky appear sliglitly tinted wi^h yellow, from the 

 horizon to a certain height. In those circumstances all pho- 

 tographers must have observed that the operation in the ca- 

 mera obscura is excessively slow. 



This may probably explain those dark specimens*, which 

 were so weak in effect, obtained some years ago on the Alps 

 during clear and cloudless weather, by an operator sent to 

 Italy by M.- Lerebours to take Daguerreotype views. M. 

 Lerebours' operator was an experienced photographer, and 

 he found, to his great surprise, that on the summit of the Alps 

 he could not obtain any effect in less than five or six times 

 what he considered to be the necessary exposure. He was 

 surroundeil with snow, the sun was shining in all its brilliancy, 

 the sky was pure and cloudless, of a deep blue colou'*, whereas 

 the horizon was without doubt slightly tinted yellow, on ac- 

 count of the vapours produced by the melting of the snow 

 and ice of the glaciers. 



Mr. Lerebours' operator was using at that time (in 1840) 

 plates only iodized; and this fact would confirm the neutrali- 

 zation of the photogenic etfect upon plates of iodide of silver, 

 by the less refrangible rays acting simultaneously with white 

 light, acct)rcling to Dr. Draper's observations. 



It would be interesting if M. Lerebours could state the 



• I have one of tlicse curious s|jec'iiiieii.s in my possession, for uhicli 1 am 

 indebted to M. I.erebuiirs' kindness. 



