Prof. Draper on the Chemical Action of Light. 385 



tlieir equilibrium is overset, it follows that whenever the circuiu- 

 stances of the experiment change, the chemical effect should be 

 increased, diminished, or altogether put an end to. Two rays 

 may be so placed in relation to one another that their motions 

 may conspire or antagonize ; and as one or other of these con- 

 ditions ensues, the chemical result should correspond. 



Our investigation therefore commences with a case of this 

 kind. Out of many that might be selected I take the instance 

 of the Daguerreotype plate, a surface which can always be ])ro- 

 duced uniform and pure. It is essential that the substance used 

 in such an inquiry should always have an identical constitution. 

 For this reason the plate is not to be rendered more sensitive by 

 exposure to bromine, or other preparations, but the vapour of 

 iodine alone is to be used. It gives a surface of uniform che- 

 mical constitution, sufficiently sensitive for such purposes. 



Those who are familiar with the process know, that when a 

 polished silver plate is exposed to the fumes of iodine, the surface 

 presently tarnishes, and passing through variou^s shades of lemon- 

 and orange-yellow, turns gradually red and blue, then recovers 

 an uncolourcd metallic lustre, somewhat approaching its original 

 aspect. If the exposure be prolonged, a second series of colours 

 succeeds, commencing as before with a yellow. These different 

 tints arise as the film becomes of different thickness. 



There is no difficulty in obtaining on the same piece of polished 

 silver spaces or bands of these different tints, by screening its 

 surface in a suitable manner while it is exposed to the iodine 

 Vapour. Having therefore provided a plate of silver, I found 

 upon its surface transverse bands of different tints, yellow, rod, 

 blue, gray, yellow, &c. The plate might therefore be regarded 

 as presenting an uniform surface of pure iodide of silver, but of 

 a different thickness in the different bands, the second yellow 

 having twice the thickness of the first. , 



The production of the colour undoubtedly arises from the in- 

 terference of the incident ray with the ray reflected from the 

 metal at the back of the film. 



If now the plate, thus crossed with its tranverse bands of dif- 

 ferent tints, be exposed to an uniform light, it changes, and the 

 effect may be subsequently brought out by exposure to the vapour 

 of mcrcu)'y. IJut it at onc(! appears that there has been a great 

 difference of action upon the ditferent coloured bands; that the 

 two yellow Rpacos have been powerfully affected, the blue less, 

 and the metallic gray portion scarcely at all. It is the same 

 iodide of silver all over, and yet it presents this difference of 

 tesult. In what other way can the experiment be explained 

 than by admitting, that, in those bands that have been unapted 

 upon, th(; effect of the incident ray has been destroyed, or' re- 



Phil. May. S. 1 . Vol. 1 . No. 3. May 1851 . 2D'"' 



