DAGUERREOTYPE. 367 



by exposure to light usually does not go on any further by long 

 continued keeping in the dark, nor does it become materially 

 obliterated or modified by so doing ; so that whether the exposed 

 plate be developed at once, or not till after several weeks or even 

 months, makes little (if any) difference in the end result. 



In one of the earliest photographic processes invented (now 

 rarely if ever used), termed the Daguerreotype, after its inventor, 

 M. Daguerre, the image is formed on a dry plate surface obtained 

 by exposing a plate of silver (or copper coated with silver) to 

 iodine vapour, whereby a film of iodide of silver is formed by 

 direct combination (Chapter XIV.) of silver and iodine. After 

 exposure in the camera little or no trace of a picture is visible ; but 

 a latent image exists, capable of being developed in a peculiar way. 

 The exposed plate is placed in contact with hot mercury vapour, 

 so that mercury may condense on its surface ; wherever the light 

 has acted most, the mercury condenses most readily, the altered 

 silver iodide exerting a kind of attraction for the mercury, some- 

 what akin to that exerted by particles of metallic silver upon a 

 mixture from which silver is being reduced (Expt. 391) ; whilst 

 the unaltered silver iodide does not attract the mercury at all. 

 Finally, the unaltered iodide of silver is dissolved away, and the 

 picture fixed by means of sodium thiosulphate ; so that when the 

 picture is held in a proper position with respect to the light, the 

 condensed quicksilver appears white or greyish white by contrast 

 with the rest of the ground, thus giving a positive picture. 



Expt. 391. Intensification and Reducing. When an exposed 

 plate is developed it sometimes happens that the lights and 

 shadows are too strongly contrasted, owing to the reduction of 

 silver in too great quantity, more especially when the action of the 

 light has been somewhat too vigorous. A negative which is thus 

 too dense, can sometimes be improved by subjecting it to the action 

 of some substance that will dissolve away a portion of the deposited 

 silver and thus thin down or " reduce " the image. This can be 

 done by immersion in dilute nitric acid ; preferably less energetic 

 agents are employed, such as ferric chloride, ferriryanide of 

 potassium, or other substance that will partly convert the deposited 

 silver into chloride (or bromide, ferricyanide, &c.), which can 

 then be dissolved out by the same reagents as are used for fixing 

 (cyanide of potassium or thiosulphate of sodium). 



On the other hand, when an image is too faint it requires 

 intensification. In the case of wet collodion plates this is usually 

 effected by pouring on to the plate a solution of silver nitrate 

 mixed with ferrous sulphate or other reducing agent ; this mixed 

 fluid tends to deposit metallic silver, but the particles of silver 



