The materials and apparatus necessary for the Calotype process are--

Two or Three Shallow Dishes, for holding distilled water, iodide, potassium, &c.--the same water never being used for two different operations.

White Bibulous Paper.

Photogenic Camera--Fig. 9.

Pressure Frame--Fig 29.

Paper, of the very best quality--directions for the choice of which have been already given.

A Screen of Yellow Glass.

Camels' or Badgers' hair Brushes:--A seperate one being kept for each wash and solution, and which should be thoroughly cleansed immediately after using in distilled water. That used for the gallo-nitrate is soon destroyed, owing to the rapid decomposition of that preparation.

A Graduated Measure.

Three or Four Flat Boards, to which the paper may be fixed with drawing pins.

A Hot Water Drying Apparatus, for drying the paper will also be found useful.

In preparing the Calotype paper, it is necessary to be extremely careful, not only to prevent the daylight from impringing upon it, but also to exclude, if possible, the strong glare of the candle or lamp. This may be effected by using a shade of yellow glass or gauze, which must be placed around the light. Light passing through such a medium will scarcely affect the sensitive compounds, the yellow glass intercepting the chemical rays.

Preparation of the Iodized Paper.--Dissolve one hundred grains of crystalized nitrate of silver in six ounces of distilled water, and having fixed the paper to one of the boards, brush it over with a soft brush on one side only with this solution, a mark being placed on that side whereby it may be known. When nearly dry dip it into a solution of iodide of potassium, containing five hundred grains of that salt dissolved in a pint of water. When perfectly saturated with this solution, it should be washed in distilled water, drained and allowed to dry. This is the first part of the process, and the paper so prepared is called iodized paper. It should be kept in a port-folio or drawer until required: with this care it may be preserved for any length of time without spoiling or undergoing any change.

Mr. Cundell finds a stronger solution of nitrate of silver preferable, and employs thirty grains to the ounce of distilled water: he also adds fifty grains of common salt to the iodide of potassium, which he applies to the marked side of the paper only. This is the first process.

Preparation of the paper for the Camera.--The second process consists in applying to the above a solution which has been named by Mr. Talbot the "Gallo-Nitrate of Silver;" it is prepared in the following manner: Dissolve one hundred grains of crystalized nitrate of silver in two ounces of distilled water, to which is added two and two-third drachms of strong acetic acid. This solution should be kept in a bottle carefully excluded from the light. Now, make a solution of gallic acid in cold distilled water: the quantity dissolved is very small. When it is required to take a picture, the two liquids above described should be mixed together in equal quantities; but as it speedily undergoes decomposition, and will not keep good for many minutes, only just sufficient for the time should be prepared, and that used without delay. It is also well not to make much of the gallic acid solution, as it will not keep for more than a few days without spoiling. A sheet of the iodized paper should be washed over with a brush with this mixed solution, care being taken that it be applied to the marked side. This operation must be performed by candle light. Let the paper rest half a minute, then dip it into one of the dishes of water, passing it beneath the surface several times; it is now allowed to drain, and dried by placing its marked side upwards, on the drying apparatus. It is better not to touch the surface with bibulous paper. It is now highly sensitive, and ready to receive the impression. In practice it is found better and more economical not to mix the nitrate of silver and gallic acid, but only to brush the paper with the solution of the nitrate.

Mr. Talbot has recently proposed some modifications in his method of preparing the calotype paper. The paper is first iodized in the usual way; it is then washed over with a saturated solution of gallic acid in distilled water and dried. Thus prepared he calls it the io-gallic paper: it will remain good for a considerable time if kept in a press or portfolio. When required for use, it is washed with a solution of nitrate of silver (fifty grains to the ounce of distilled water), and it is then fit for the camera.

Exposure in the Camera.--The calotype paper thus prepared possesses a very high degree of sensibility when exposed to light, and we are thus provided with a medium by which, with the aid of the photogenic camera, we may effectually copy views from nature, figures, buildings, and even take portraits from the shadows thrown on the paper by the living face. The paper may be used somewhat damp. The best plan for fixing it in the camera is to place it between a piece of plate glass and some other material with a flat surface, as a piece of smooth slate or an iron plate, which latter, if made warm, renders the paper more sensitive, and consequently the picture is obtained more rapidly.