SNOW CRYSTALS, PHOTOGRAPHING.—Snow crystals evaporate rapidly even in low temperature, and the work requires to be done rapidly and with caution. Freshly fallen snow only will give a good photograph, and as we are compelled to work in the midst of the snow storm, the task becomes still more complicated and difficult. Snow crystals but a short time after falling break, the broken pieces freeze together and crystallization is destroyed. For the illumination of snow crystals, transmitted light only can be used, reflected light destroys the shadows, and injures the high lights, and the result is necessarily but a very imperfect picture of the object.
Diffused light, especially that of a dark winter's day, and during a snow storm, is not fit for this kind of photo-micrographic work, and we must resort to artificial light, preferably to that of a petroleum lamp. To prevent heat action emanating from the illuminating ray cone, an ab sorptive cell of alum solution should be interposed. As alum solution freezes solid in —5 deg. R., chloride of sodium is added. With Hartnark's projection system = 31 mm. focus distance, from 5 to 7 seconds upon an erythrosine plate is ample.
Dr. Neuhaus has made photographs of more than 6o different ice and snow specimens. The pictures of ice crystals much resemble those of hoar frost, deposited after a cold winter's night. Of snow crystals, the doublets are highly interesting, two crystals merged into one, and those having passed through a moist stratum of air, when microscopic drops of water will freeze into the hexagonal form, giving the picture an appearance very much resembling cauliflower.
SNOW PHOTOGRAPHY.—The photographing of snow scenes with natural and brilliant contrasts of black and white is not by any means an easy task. Many authorities have stated
that snow scenes require sunlight, otherwise the result will be flat. Captain Abney, however, in photographing in the Alps, found that the color values of the landscape are often more correctly rendered as the light declines. His method is to expose four or five times longer than is usually done, so that there is a reserve of power, as it were, during the actual process of development, which is as follows: (a.) Give plenty of alkali with very little pyro, and you get a feeble image; then increase the pyro, and you get density.
(b.) Add plenty of bromide, three or four times more than is ordinarily recommended, more particularly when there has been a very full exposure.
(c ) The image appears very, very gradually; indeed, before all the detail is apparent, a quarter of an hour elapses. When viewed by transmitted light the image will be a phantom one, and, if fixed, would be unprintable.
(d.) The next operationjust reverses these principles; the developer is poured off, and pyro (this time a good quantity) and a little bromide are added to two ounces of water, and the necessary density is rapidly obtained, occasionally adding two or three drops of a ten per cent. solution of ammonia.
SOAP.—Combinations of the alkalies, potash and soda, with fatty acids, are what are usu ally termed soaps.
The soaps used in photography are—Castile soap (q.v.), used as a lubricator in burnishing; cyanogen soap, used for removing silver stains from the hands; and " Monkey" soap for cleaning glass or other materials.
SODA.—Common washing soda is an impure carbonate of soda (q. v.) SODA, BICARBONATE OF.—See Sodium Bicarbonate.
SODA DEVELOPER.—See Developer.