Mr. Martyn Roberts 07i Daguerreotype. 301 



LI. On the Cause of the Production of Daguerreotype Pictures. 

 By Martyn J. Roberts, Esq. 



To the Editors of the Philosophical Magazine and Journal. 



Gentlemen, 

 A LTHOUGH the Daguerreotype process has long excited 

 ■^^ intense interest in the scientific world, we have not as 

 yet had an explanation of the wonderful effects produced. I 

 believe that even M. Arago has failed to elucidate the theory; 

 and not only has this great philosopher been foiled, but the in- 

 ventor of the process is unable to solve the problem ; with 

 these facts before me, I feel it almost presumptuous in so hum- 

 ble a votary of science as myself to attempt an explanation 

 of the cause of the production of Daguerreotype pictures, but if 

 my attempt has the effect of directing attention to the right 

 path for arriving at a solution of the question, I shall be sa- 

 tisfied. We all know that light has a powerful influence on 

 crystallization : solutions that will not crystallize in the dark 

 instantly form on the admission of light; the crop of crystals 

 is always more copious on the enlightened side of a glass 

 containing a crystallizing liquid than on the dark side. Ice 

 forms more rapidly during moonlight, and on the break of 

 day, than on a dark night; but I need not adduce examples 

 of the influence of light on crystallization, for the fact is 

 allowed by all scientific men. 



Let us then suppose, that in the Daguerreotype process the 

 cleansed silver plate is exposed in the dark to the vapour of 

 iodine; this deposits itself in a flocculent or powdery state 

 on the plate, unable to form the peculiarly shaped iodic cry- 

 stals, from the absence of light ; but yet all other requisites 

 being present, it may be considered in an incipient state of 

 crystallization, or balanced so finely, that the admission of the 

 excitant light instandy throws it into plate-formed iodic cry- 

 stals, but only in those parts where the light has impinged, 

 and here its perfection of, or continuity of crystallization, is 

 merely in proportion to the intensity of light. 



Having now the iodated plate removed from the camera 

 obscura, where it has undergone a surface crystallization, 

 more or less perfect in those parts where the lights and shades 

 have fallen, we submit it to the mercurial vapour; the atoms, 

 vesicles, or globules of this vapour being very minute, attach 

 themselves to all the minute inequalities of face in the iodine; 

 on those parts which are fully crystallized, Uie vapour is pre- 

 cipitated on the flat tabular surface of the crystals, and here 

 offering a continuous and equal angle of reflection to the eye 



