The Dictionary Of Photography

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

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based upon the action of light upon bromide of silver paper impregnated with solution of nitrite of potassium. There are such actinometers in the market, as Stanley's and Watkins', in which the actinic power of the light is gauged by the time a bromide of silver paper prepared as above takes to match a standard tint. In connection with this we would point out that it is said to be extremely difficult to match the standard tint; but the following quotation from the instructions issued by the maker of the latter actinometer is worth consideration : - " In testing the light no notice should be taken of the exact colour of the sensitive paper, which may vary slightly with the humidity of the air; the depth of tint is the important point. The paper darkens rapidly in light; up to a certain point it is lighter than the standard tint, after this point it is darker. The point when it is neither lighter nor darker is that to be timed." To prepare a somewhat similar actinometer it is only necessary to soak ordinary bromide paper in a 10 per cent, solution of nitrite of potassium, and then to dry it in the dark. A small piece is allowed to darken in daylight, and the time that it takes to deepen to a tint which may be arbitrarily chosen accurately counted, or timed by a watch. This tint should then be matched in water-colours, and painted on a strip of paper. It must be noted, however, that the tint of the paint when dry must agree with the tint of the darkened paper. Now, to use this actinometer, paste the strip of painted paper on the top of a cardboard match-box, place inside the box the sensitive paper soaked in the nitrite solution, and draw a small piece out and allow it to darken at a distance of about 18 ins. from a No. 5 Bray's gas burner turned full on without flaring behind the half-tones of a negative, noting accurately the number of seconds it takes to darken to the standard tint. Now take a sample of some bromide paper, Eastman's, for instance, and expose half-a-dozen sheets of the same paper behind the negative at the same distance, 18 ins., giving various exposures, then on developing these six sheets it will be possible to pick out one print correctly exposed; and from this we can establish one factor, which will enable us to calculate other exposures under other conditions. Thus if the actinometer paper takes 10 seconds to darken to the standard tint, and we find 8 seconds the correct exposure for the said negative at 18 ins. from the gas burner, it will not be