Photo by Martin.
FIG. 64.—A NATURAL PHENOMENON IN ICE.
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 absorptive cell of alum solution should be interposed. As alum solution freezes at about 20° Fahr., chloride of sodium is added. With Hartnark's projection system, at 31 mm. focus distance, from 5 to 7 seconds upon an erythrosine plate is ample.
FIG. 65.—PHOTOGRAPH OF FROST CRYSTALS.
BY JAS. LEADBEATER.
Dr. Neuhaus has made photographs of more than 60 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.
FIG. 66.—PHOTOGRAPH OF FROST.
BY JAS. LEADBEATER.
The most difficult question of all remains, the cause of the various forms of the hexagonal crystals, which frequently change in the same snowfall. Instead of advancing a new hypothesis, says Hellmann, it is better to acknowledge that we know nothing positively in regard to this. In our knowledge of the form and structure of the snow we have made great advance since the time of Kepler, but after nearly four hundred years, we cannot give a satisfactory answer to his question, "Cur autem sexangula? Why six-sided?"
We do not know the special conditions which determine the formation of one or the other form of snow crystals. We have found that a low temperature favors the formation of tabular crystals; a higher temperature the star shaped crystals; these groups show such multifarious forms that it is necessary to seek for other causes which influence the formation of snow figures. There is offered here a broad field for new investigation and study.
We give a reproduction (Fig. 64) of a photograph of a curious group of crystals. Some water had been left in a 10×8 dish on a winter day, and a film of ice was seen floating on the surface. The formation of the crystals and the floral design were so beautiful that it was taken out and photographed. The delicate lace-like edging of the glacial tracery is the result of the deposition of hoar frost while draining off the water from the ice leaves and flowers and fixing the image in the camera.