PROTOGENIC DRAWING. 



159 



sion; performs at once the functions of 

 the eye and of the optic nerve — the 

 material instrument of sensation and 

 sensation itself ? " 



Description of Photofi;enic Draw- 

 ing. — The subject (says Mr. Talbot) na- 

 turally divides itself into two heads — the 

 preparation of the paper, and the means 

 of fixing the design. In order to make 

 what may be called ordinary photogenic 

 paper, the author selects in the first place, 

 paper of a good firm quality, and smooth 

 surface; and thinks, that none answers 

 better than superfine writing paper. He 

 dips it in a iveak solution of common 

 salt, and wipes it dry, by which the salt 

 is unifornily distributed throughout its 

 substance. He then spreads a solution 

 of nitrate of silver on one surface only, 

 and dries it at the fire. The solution 

 should not be saturated, but six or eight 

 times diluted with v^'ater. When dry, 

 the paper is fit for use. He has found, 

 by experiment, that there is a certain 

 proportion between the quantity of salt 

 and that of the solution of silver which 

 answers best, and gives the maximum 

 effect. If the strength of the salt is 

 augmented beyond this point, the effect 

 diminishes, and in certain cases becomes 

 exceedingly small. This paper, if pro- 

 perly made, is very useful for all ordinary 

 photogenic purposes. For example, no- 

 thing can be more perfect than the images 

 it gives of leaves and flowers, especially 

 with a summer's sun. The light passing 

 through the leaves, delineates every 

 ramification of their nerves. If a sheet 

 of paper, thus prepared, be taken and 

 washed with a saturated solution of salt 

 and then dried, it will be found (especially 

 if the paper has been kept some weeks 

 before the trial, is made,) that its sensibili- 

 ty is greatly diminished, and ip some 

 cases, seems quite extinct. But if it be 

 again washed with a liberal quantity of 

 the solution of silver, it becomes again 

 sensible to light, and even more so than 

 it was at first. In this way by alternately 

 washing the pa[)er with salt and silver, 

 and drying it between times, M, Talbot 

 has succeeded in increasing its sensibility 

 to the degree that is requisite for receiv- 

 ing the images of the camera obscura. 

 In conducting this operation, it will be 



found that the results are sometimes more, 

 and sometimes less satisfactory, inconse- 

 quence of small and accidental variations 

 in the proportions employed. It happens 

 sometimes that the chloride of silver is 

 disposed to darken of itself, without any 

 exposure to the light. This shows that 

 the attempt to give it sensibility has been 

 carried too far. The object is, to ap- 

 proach to this condition as near as possi- 

 ble, without reaching it; so that the sub- 

 stance may be in a state ready to yield 

 to the slightest extraneous force, such as 

 the feeble impact of the violet rays, when 

 much attenuated. Having, therefore, pre- 

 pared a number of sheets of paper, slight- 

 ly different from one another in the com- 

 position, let a piece be cut from each, and, 

 having been duly marked or numbered, 

 let them be placed side by side in a very 

 weak diffused light, for about a quarter 

 of an hour; then, if any one of them, as 

 frequently happens, exhibits a marked 

 advantage over its competitors, M.Talbot 

 selects the paper which bears the cor- 

 responding number to be placed in the 

 camera obscura. 



With regard to the second object — that 

 of fixing the images — M.Talbot observed, 

 that, after having tried ammonia, and 

 several other re-agents with very imper- 

 fect success, the first which gave him a 

 successful result, was the iodide of potas- 

 sium, much diluted with water. If a 

 photogenic picture is washed over with 

 this liquid, an iodide of silver is formed, 

 which is absolutely unalterable by sun- 

 shine. 



This process requires precaution; for, 

 if the solution is too strong, it attacks the 

 dark parts of the picture, it is requisite, 

 therefore, to find, by trial, the proper 

 proportions. The fixation of the pictures 

 in this way, with proper management, is 

 very beautiful and lasting. The specimen 

 of lace which JSI. Talbot exhibited to the 

 society, and which was m.ade five years 

 ago, was preserved in this manner. But 

 his usual method of fixing is different 

 from this, and somewhat simpler, or, at 

 least, requiring less nicety. It consists 

 in immersing the picture in a strong so- 

 lution of common salt, and then wiping 

 off the superfluous moisture, and dry- 

 ing it. 



