The Barnet Book of Photography: A Collection of Practical Articles. Various

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СКАЧАТЬ dry. S Saturated solution of sulphite of soda.

      

      When the view has strong contrasts and the plate has been exposed for the shadows take of A 30 minims and 2 ounces of water and soak the plate in it as given above. Then add to the cup, of B 2 drams, of S 1 dram, and about quarter grain of P. Pour back the solution of ammonia from the dish, and then apply the mixture till all detail appears, and proceed as indicated above. A saturated solution of potassium carbonate may be substituted for the ammonia solution.

      Before closing this chapter it may be of use to the reader to tabulate the number of thicknesses of atmospheres through which light has to travel at different altitudes of the sun at sea level.

Altitude.				Atmosphere.
	90°			1.000
	80°			1.015
	70°			1.064
	60°			1.155
	50°			1.305
	40°			1.555
	30°			1.995
	20°			2.904
	15°			3.809
	10°			5.571
	5°			10.216
	4°			12.151
	2°			18.882
	0°			35.503

      If sunlight outside the atmosphere be represented by 1 and say ¹/₁₀th be cut off by 1 atmosphere, then after transmission through 2 atmospheres only .81 will reach the spectator, and if through 3 only .729. For any atmosphere the diminution will be ¹/₁₀th, that is, it will be .9^x where x is the number of atmospheres.

      If we ascend the factor varies, there are less thicknesses of atmosphere to go through and we get the following table.

      

Barometer in Inches.	Visual Transmission Sunlight outside the Atmospheric being 1).	Photographically Actinic Light Transmitted (Sunlight outside the Atmospheric being 1).
30	.853	.639
29	.866	.654
28	.875	.672
27	.884	.689
26	.891	.708
25	.899	.730
24	.908	.746
23	.915	.763
22	.922	.787
21	.928	.800
20	.934	.819
19	.940	.833

      This table and the preceding one will enable a calculation to be made as to the exposure to be given. Thus at sea level with a photographic brightness of sun of 639,000 candles when nearly overhead, it will at 5° above the horizon only have a photographic brightness of about 1000. At about 9000 feet high the photographic brightness would when the sun is overhead be about 800,000 candles, and at 5° it would have a value of 350,000, showing the greater penetration through the thinner atmosphere.

      W. de W. Abney, C.B., F.R.S., etc., etc.

Negative Making.

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