We shall now proceed to describe the various processes for Photogenic drawing on paper; first, however, impressing on the mind of the experimenter, the necessity which exists for extreme care in every stage of the manipulation. In this portion of my work I am entirely indebted to the works of Professors Hunt, Fisher and others.

I. APPARATUS AND MATERIALS.--Paper.--The principal difficulty to be contended with in using paper, is the different power of imbibition which we often find possessed in the same sheet, owing to trifling inequalities in its texture. This is, to a certain extent, to be overcome by a careful examination of each sheet, by the light of a candle or lamp at night, or in the dark. By extending each sheet between the light and the eye, and slowly moving it up and down, and from left to right, the variations in its texture will be seen by the different quantities of light which pass through it in different parts; and it is always the safest course to reject every sheet in which inequalities exist. Paper sometimes contains minute portions of thread, black or brown specks, and other imperfections, all of which materially interfere with the process. Some paper has an artificial substance given to it by sulphate of lime (Plaster of Paris); this defect only exists, however, in the cheaper sorts of demy, and therefore can be easily avoided. In all cases such paper should be rejected, as no really sensitive material can be obtained with it. Paper-makers, as is well known, often affix their name to one half the sheet; this moiety should also be placed aside, as the letters must frequently come out with annoying distinctness. Well sized paper is by no means objectionable, indeed, is rather to be preferred, since the size tends to exalt the sensitive powers of the silver. The principal thing to be avoided, is the absorption of the sensitive solution into the pores; and it must be evident that this desideratum cannot be obtained by unsized paper. Taking all things into consideration, the paper known as satin post would appear to be preferable, although the precautions already recommended should be taken in its selection.

Brushes.--The necessary solutions are to be laid upon the paper by brushes. Some persons pass the paper over the surface of the solutions, thus licking up, as it were, a portion of the fluid; but this method is apt to give an uneven surface; it also rapidly spoils the solutions. At all events, the brush is the most ready and the most effectual means.

Distilled Water.--All the water used, both for mixing the solutions, washing the paper, or cleaning the brushes, must be distilled, to obtain good results, for reasons before specified.

Blotting Paper.--In many instances, the prepared paper requires to be lightly dried with bibulous paper. The best description is the white sort. In each stage of the preparation distinct portions of bibulous paper must be used. If these be kept seperate and marked, they can be again employed for the same stage; but it would not do, for example, to dry the finished picture in the same folds in which the sensitive paper had been pressed. A very convenient method is to have two or three quarto size books of bibulous paper, one for each seperate process.

Nitrate of Silver.--In the practice of the photographic art, much depends on the nitrate of silver. Care should be taken to procure the best; the crystalized salt is most suitable for the purpose. While in the form of crystal it is not injured by exposure to light, but the bottles containing the solutions of this salt should at all times be kept wrapped in dark paper, and excluded from daylight.

II. DIFFERENT METHODS OF PREPARING THE PAPER.--Preparation of the Paper.--Dip the paper to be prepared into a weak solution of common salt. The solution should not be saturated, but six or eight times diluted with water. When perfectly moistened, wipe it dry with a towel, or press it between bibulous paper, by which operation the salt is uniformly dispersed through its substance. Then brush over it, on one side only, a solution of nitrate of silver. The strength of this solution must vary according to the color and sensitiveness required. Mr. Talbot recommends about fifty grains of the salt to an ounce of distilled water. Some advise twenty grains only, while others say eighty grains to the ounce. When dried in a dark room, the paper is fit for use. To render this paper still more sensitive, it must again be washed with salt and water, and afterwards with the same solution of nitrate of silver, drying it between times. This paper, if carefully made, is very useful for all ordinary photographic purposes. For example, nothing 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 fibres. In conducting this operation, however, it will be found that the results are sometimes more and sometimes less satisfactory, in consequence of small and accidental variations in the proportions employed. It happens sometimes that the chloride of silver formed on the surface of the paper is disposed to blacken of itself, without any exposure to light. This shows that the attempt to give it sensibility has been carried too far. The object is, to approach as nearly to this condition as possible without reaching it; so that the preparation may be in a state ready to yield to the slightest extraneous force, such as the feeblest effect of light.