Page:Popular Science Monthly Volume 53.djvu/89

Rh The illustrations which accompany this article are all of them photomicrographs taken directly from crystals which were collected in northern Vermont during the past fifteen years. They are selected from over five hundred different forms. A close and minute study of many of them will reveal beauty and complexity of structure not seen by a casual observer. The methods employed in obtaining the illustrations have been very simple. It has been found that any apparatus which can be used for taking photomicrographs will serve to photograph snow crystals, but the microscope should be fitted with half-inch or two-thirds-inch objectives, of wide aperture and short axis; the focusing arrangement must work (juickly and accurately; the diaphragm aperture be small, not more than one-sixteenth inch; the illumination ordinary, uncondensed daylight; the exposure, rapid plates being used, from forty seconds to three hundred, as the light is strong or weak, camera bellows closed or elongated, etc. A black card placed between two pins which project from the stage on each side of the objective serves to exclude unwelcome light when the slides of the plate-holder are changed. Great care is necessary to prevent the crystals from melting, as this is one of the chief difficulties which must be overcome. On this account the observer must not breathe upon his slides, nor liandle them except with gloved hands. The whole work must be done in a cold room, with but one unscreened window. Crystals may be collected as they fall, upon a black card, and transferred by a bit of broom splint to a glass slide upon which they may be pressed flat with a feather.

Careful examination of the illustrations will soon convince one that, great as is the charm of outline, the internal ornamentation of snow crystals is far more wonderful and varied. Many of the specimens, we might almost say all of them, exhibit in their interior most fascinating arrangements of loops, lines, dots, and other figures in endless variety. So far as is known to the writer, these illustrations are the first which have been published that show in any adequate manner these interior figures, and surely they add greatly to our interest and delight as we study snow crystals. So varied are these figures that, although it is not difficult to find two or more crystals which are nearly if not quite the same in outline, it is almost impossible to find two which correspond exactly in their interior figures.

It is asserted by some observers that many of the lines or rods seen in the interior of snow crystals are really tubes filled with air.

Perfect crystals are by no means always common in snow storms, most of the forms produced being more or less unsymmetrical or otherwise imperfect. It rarely happens that during a single winter there are more than a dozen good opportunities for securing