C 374 ] 



XLVII. Researches on the Theory of the principal Phcenomena 

 of Photography in the Daguerreotype Process. By A. Clau- 



DET*. 



A LTHOUGH the Daguerreotype process has during the 

 ■^^ last ten years been investigated by a great number of 

 philosophers, and brought to a considerable degree of perfec- 

 tion by a still greater number of practitioners, it may appear 

 surprising that the principal phaenomena upon which this new 

 art is founded, are still enveloped in a mysterious darkness. 



My constant endeavour has been to explain them, and at 

 the tv/o last meetings of the British Association I have had 

 the honour of communicating the results of some of my re- 

 searches. 



The phaenomena which have not yet been satisfactorily 

 explained, and of which 1 shall have to treat in the present 

 paper, are those referring to the following points : — 



1. What is the action of light on the sensitive coating? 



2. How does the mercurial vapour produce the Daguer- 

 reotype image ? 



3. Which are the particular rays of light that impart to the 

 chemical surface the affinity for mercury ? 



4. What is the cause of the difference in achromatic lenses 

 between the visual and photogenic foci? why do they con- 

 stantly vary ? 



5. What are the means of measuring the photogenic rays, 

 and of finding the true focus at which they produce the image ? 



At the last meeting of the British Association, which took 

 place at Swansea, I announced that the decomposition of the 

 chemical surface of the Daguerreotype plate by the action of 

 certain rays of light produced on that surface a white precipi- 

 tate, insoluble in the hyposulphite of soda, which, when ex- 

 amined by the microscope, had the appearance of crystals re- 

 flecting light, and which, when seen by the naked eye, were 

 the cause of a positive Daguerreotype image. 



This fact had not been observed before. The opinion of 

 Daguerre himself and other writers was, that the action of 

 light on the iodide of silver had only the effect of darkening 

 the surface, and consequently of producing a negative image. 

 But it escaped them, that, under the darkened iodide of silver, 

 another action could take place after a continued exposure to 

 light, and that the hyposulphite of soda washing could dis- 

 close a positive image. I have proved this unexpected fact in 

 obtaining, by the action of light only, and without mercury, 



* Communicated by the Author, having been read before the British 

 Association at Birmingham, Sept. 14, 1849. 



