Page:Popular Science Monthly Volume 31.djvu/734

716 are imaginary; and that while the main source of light should always be on the left, it might be advantageously supplemented by lights of lesser intensity from either side, whereby the total illumination would be increased. In wide rooms the rows of desks on either side should face in opposite directions, so that the nearer and consequently stronger light should always come from the left. No natural light not coming direct from the sky could be sufficient, but whether sky-light were so or not would depend on the angle of aperture, or arc of the sky visible at any given point, which should never be less than 5°, and the angle of incidence, which should not be less than 25°. In artificial lighting, shades acting also as reflectors are preferable to semi-opaque globes, which involve much loss of light. The two points to be observed in the arrangement of the lighting are the avoidance of shadows and of the direct incidence of the light on the eyes.

Uses of Birch-Bark.—Professor Gustav Retzius has a chapter, in his sketches of Finland, respecting the uses which the Finnish people make of birch-bark. Shoes are made of it. The bark, having been peeled, is cut into strips, rolled up carefully, and put away for future use. When the Finnish peasant wants a pair of new shoes, he takes one of his rolls, cuts it up into strips of suitable width, soaks it in water to soften it, and then weaves it into the form he desires. It is all done very rapidly, a half-hour or less sometimes sufficing for the whole work. The shape of the shoes varies according to the use they are to be put to. If they are for swamps, he makes them into low-cut slippers or sandals. With these he can walk through the moors without wounding his feet on the sharp sticks, while the water flows in and out freely. Another shape is that of regular shoes with high sides, and toes either run out to a point or cut short off. They are made wider than is necessary to accommodate the foot, so that in cold weather they may be stuffed with hay and the foot wrapped with bandages. A third form is a kind of half-boot. All the three forms are still used in much of the interior of the country during work in the fields, the swamps, the woods, and the house, especially among the poorer people. These bark shoes have in fact many advantages. They are very cheap, costing really nothing; they are strong and durable, and warm, with the help of hay and wrappings. In moist ground they let the water in and out without softening, and, by changing the hay or wrappings, are easily dried. Many other uses are found for birch-bark. The sheath in which the peasant carries his inseparable knife is made of it. It is woven into pockets or bags of various sizes, which are used all over Finland. The larger of these form a satchel with a flap which may be turned over and close the bag. Wherever one may travel in Finland, he will meet children, women, and men with these satchels. The peasant carries his dinner in one, and with the satchel on his back, birch-bark shoes on his feet, and his tools in his hands, goes out equipped for his day's work. These satchels also take the place of baskets. Birch-bark is made into salt-tubs, cords and lines, brushes for washing out wooden vessels, boxes, sieves and tubs, and many other articles for which we use wood or basket material.

The Microscope in Geology.—President T. G. Bonney, of the Geological Section of the British Association, spoke on the "Application of Microscopic Analysis to discovering the Physical Geography of by-gone Ages." The microscope furnishes us with an instrument of precision, by means of which we can learn the more minute mineral composition and structural peculiarities of rocks, can recognize fragments, and sometimes even determine the source of the smaller constituents in a composite elastic rock. Thus, by its aid, we may be able, in many cases, to substitute a demonstration for a conjecture. The speaker described the materials of the coarser fragmental rocks of Great Britain, and laid down the following principles of interpretation: 1. Pebbles indicate the action either of waves of the sea, or of strong currents, marine or fluviatile. 2. The zone in the sea over which the manufacture of pebbles can be carried on is seldom wider than from the high-tide line to a depth of within twenty feet below low-water mark. It is, therefore, probable that a thick and very