HISTORY OF PHOTOGRAPHY. 179 



paper. This action appears to consist in tlie disengagement of free chromic 

 acid, wliicli is of a deep red color and which seems to combine with the paper. 

 This is rendered more probable from the circumstance that the neutral chromate 

 exhibits no similar change. The best mode of preparing paper with bichromate 

 of potash is to use a saturated solution of that salt, soak the paper well in it, 

 and then dry it rapidly at a brisk fire, excluding it from daylight. Paper thus 

 prepared acquires a deep orange tint on exposure to the sun. If the solution 

 be less strong or the drying less rapid, the color will not be so deep. A pleas- 

 ing variety may be made by using sulphate of indigo along with the bichromate 

 of potash, the color of the object and of the paper being then different shades 

 of green. In this way, also, the object may be represented of a darker shade 

 than the ground. 



Paper prepared with the bichromate of potash, though as sensi- 

 tive as some of the papers prepared with the salts of silver, is much 

 inferior to most of them and is not sufficiently sensitive for the 

 camera obscura. This paper, however, answers quite well for taking 

 drawiaigs from dried plants or for copying prints. Its great recom- 

 mendation is its cheapness and the facility with which it can be pre- 

 pared. The price of the bichromate of potash is about 2 shillings 

 per pound, while the nitrate of silver is 5 shillings an ounce. 



As the deep orange ground of these pictures prevents the per- 

 meation of the chemical rays of light, it is very easy to procure any 

 nimiber of facsimiles of an engraving by transfer from the first 

 negative photograph. The correct copies have a beautiful sharpness, 

 and, if carefully managed, but little of the minute detail of the origi- 

 nal engraving is lost. 



A photographic paper prepared with the bichromate of potash 

 of another kind is described by M. E. Becquerel. He states : It is 

 sufficient to steep a paper prepared in Mr. Ponton's manner, and uj)on 

 which there exists a faint copy of a drawing, in a solution of iodine in 

 alcohol, to wash this paper in alcohol, and then dry it; then the 

 parts which were white become blue, and those which were yellow 

 remain more or less clear. 



M. E. Becquerel has pursued his investigations into the action of 

 the chromic acid on organic compounds, and has shown that the 

 mode of sizing the papers influences their coloration by light, and 

 that with unsized paper coloration is effected only after a long time. 

 Perceiving that the principal reaction resulted from the chromic 

 acid contained in the bichromate of potash on the starch in the size 

 of the paper, it occurred to M. E. Becquerel that as starch has the 

 property of forming with iodine a combination of a very fine blue 

 color, it should produce deep shades of that tint, while the lights 

 still remain an orange yellow. 



His method of proceeding is to spread a size of starch very uni- 

 formly over the surface of the paper. It is then steeped in a weak 

 alcoholic solution of iodine, and afterwards washed in a great 



