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HISTORICAL SURVEY 15
which is still sensitive to light, must be removed to render the image durable — that is, "to fix it." This is effected by using a solution of hypo-sulphite of soda, which dissolves the iodide of silver. Nothing more is required after this than to wash with water and dry, and the daguerreotype is completed. Sometimes, in order to protect the picture, it was customary to gild it. A solution of chloride of gold was poured over, and then it was warmed ; a thin film of gold was deposited, which contributed essentially to the durability of the pictures. A picture of this nature, however, is easily injured, and requires the protection of frame and glass.
Daguerre's first pictures needed an exposure of twenty minutes — too long for taking portraits. But soon after it was found that bromine, an element having many points of resemblance with iodine, employed in combination with the latter, produced much more sensitive plates, which required far less time, perhaps not more than from one to two minutes, for exposure.
In the early period of photography persons were obliged to sit in the full sunlight, and allow the dazzling rays to fall directly upon the face — a torture which is clearly marked and visible on the portraits still preserved of these photographic victims, in the blackened shadows, the distorted muscles, and the half -closed eyes. These caricatures could certainly not bear any comparison with a good drawing from life, nor probably would portrait-photography have ever succeeded if it had not become possible to obtain good results in a moderated light. This was attained by the invention of a new lens — the double objective portrait lens of Professor Petzval, of Vienna.
This new lens was distinguished by the fact that it produced a much brighter picture than the old lens of Daguerre, so that it was now possible to take less dazzlingly illuminated objects. This lens was invented by Petzval in 1841. Voigtlander ground the lens according to his