The Dictionary Of Photography

A True Historic Record Of The Art & Practice Of Photography 100 Years Ago.

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Another rule for finding the same is : - Multiply 27 by the square of the focal length of the lens and by the fraction expressing the diameter of the diaphragm aperture.
Example: - Required the nearest point in focus with a lens 4^-in. focus,//10 diaphragm.
Fixing. The removal of any sensitive salt unacted upon by light or by the developer, thus rendering the negative or print unalterable by the further action of light. The usual method of effecting this in the case of the silver salts is by the solvent action of Hyposulphite or Thiosulphate of Sodium ; cyanide of potassium, sulphocyanide of potassium or ammonium, and sulphite of sodium, have also been recommended. Cyanide of potassium is more powerful than hypo, but its action on the image is so great as to deteriorate the half-tones occasionally. Hypo, then, is our chief resource; but the difficulty of completely eliminating, however, is its great drawback. For fixing negatives it is desirable to use two fixing baths, the strength being about 4 ozs. to the pint of water in each, the negative being allowed to remain for ten minutes in the first, and for about five minutes in the second ; by this a more thorough fixing of the negative is effected. For fixing prints the strength generally recommended is about half that for negatives - that is, about 2 ozs. to the pint - and sufficient liquor ammoniae or carbonate of ammonia should be added to make the solution smell faintly of ammonia. This neutralises any free acid which may be present in the solution, and prevents any loss of tone in fixing. One may safely use the same strength as for negatives with the addition of ammonia, as above described ; and lengthened experience shows the ad-vantage of doing this, as the fixing is completed in half the time, and there is less chance of loss of tone. R. E. Liesegang has suggested the use of Thiosinamine {q.v.) as a fixing agent, and it is specially adapted for prints. For plates its action is much slower than hypo, but it is said to possess the advantage of being more quickly removed. The strength suggested is 1 per cent. Captain Abney has recommended sulphite of soda as a fixing bath for prints, but as chloride of silver is much less soluble in sulphite of soda than in hyposulphite, failures have occurred. The following editorial remarks, taken from The Amateur