Page:Proceedings of the Royal Society of London Vol 4.djvu/338

Rh the recti muscles are, by means of this peculiar mechanism, enabled to act as antagonists to that muscle.

5. " An account of some recent improvements in Photography." By H. F. Talbot, Esq. F.R.S.

The author had originally intended, in giviag an account of his recent experiments in photography, to have entered into numerous details with respect to the phenomena observed ; but finding that to foUow out this plan would occupy a considerable time, he has thought that it would be best to put the Society, in the first place, in possession of the principal facts, and by so doing perhaps invite new observers into the field during the present favourable season for making experiments. He has, therefore, confined himself at present to a description of the improved photographic method, to which he has given the name of Calotype, and reserves for another occasion all remarks on the theory of th^ process.

The following is the method of obtaining the Calotype pictures.

Preparation of the Paper. — Take a sheet of the best writing paper, having a smooth surface, and a close and even texture.

The watermark, if any, should be cut off, lest it should injure the appearance of the picture. Dissolve 100 grains of crystallized nitrate of silver in six ounces of distilled water. Wash the paper wdth this solution, with a soft brush, on one side, and put a mark on that side whereby to know it again. Dry the paper cautiously at a distant fire, or else let it dry spontaneously in a dark room. When dry, or nearly so, dip it into a solution of iodide of po- tassium containing 500 grains of that salt dissolved in one pint of water, and let it stay two or three minutes in this solution. Then dip it into a vessel of water, dry it lightly with blotting-paper, and finish drying it at a fire, which will not injure it even if held pretty near : or else it may be left to dry spontaneously.

AU this is best done in the evening by candlelight. The paper so far prepared the author calls iodized paper, hechuse it has a uni- form pale yellow coating of iodide of silver. It is scarcely sensitive to light, but, nevertheless, it ought to be kept in a portfolio or a drawer, until wanted for use. It may be kept for any length of time without spoiling or undergoing any change, if protected from the light. This is the first part of the preparation of Calotype paper, and may be performed at any time. The remaining part is best deferred until shortly before the paper is wanted for use. When that time is arrived, take a sheet of the iodized paper and wash it with a liquid prepared in the following manner: — ■

Dissolve 100 grains of crystallized nitrate of silver in tv/o ounces of distilled water; add to this solution one- sixth of its volume of strong acetic acid. Let this mixture be called A.

Make a saturated solution of crystallized gallic acid in cold di- stilled water. The quantity dissolved is very small. Call this solu- tion B.

When a sheet of paper is wanted for use, mix together the liquids A and B in equal volumes, but only mix a small quantity of them at a time, because the mixture does not keep long without spoiUng. I shall call this mixture the Gallo-nitrate of silver.