206 THE PROCESS OF DAGUERREOTYPE. [MEMOIR XIV. 



MEMOIR XIV. 



EXAMINATION OF THE PROCESS OF DAGUERREOTYPE. 



NOTE ON LUNAR PHOTOGRAPHY. 



From the Philosophical Magazine, Sept., 1840; Smithsonian Contributions to 

 Knowledge, No. 180. 



CONTENTS : Description of the process. Cause of the deposition of mer- 

 cury and production of the light parts of the picture. Polishing of 

 the plate. The operation of iodizing. Effect of keeping the iodide. 

 The achromatic lens. Reduction of focal length in the non- achro- 

 matic. The development. Fixing by hyposulphite and galvanism. 

 Necessity of heating the plates. Lunar impressions. Artificial light. 

 Note on lunar photography The first photographs of the moon. 



MORE than one hundred instances are recorded in Ber- 

 zelius's Chemistry in which the agency of light brings 

 about changes in bodies; these are of all kinds: for- 

 mations of new compounds, re-arrangements of elements 

 already in union, changes of crystallographic character, 

 decompositions, and mechanical modifications. 



The process of the daguerreotype is to expose a sur- 

 face of pure silver to the action of the vapor of iodine, so 

 as to give rise to a peculiar iodide of silver, which under 

 certain circumstances is exceedingly sensitive to light. 

 The different operations of polishing, washing with nitric 

 acid, exposure to heat, etc., are only to secure a pure silver 

 surface ; the operation of hyposulphite of soda, and the 

 process, which I shall presently describe, of galvaniza- 

 tion, are to free the plate from its sensitive coating, and 

 in nowise affect the depth of the shadows, as some of 

 the French chemists at first supposed. 



There is but one part of the daguerreotype which 



