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THE POPULAR SCIENCE MONTHLY. 



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 in- 

 creased. In wide rooms the rows of desks 

 on either side should face in opposite direc- 

 tions, 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 wheth- 

 er sky-light were so or not would depend on 

 the angle of aperture, or arc of the sky visi- 

 ble at any given point, which should never 

 be less than 5, and the angle of incidence, 

 which should not be less than 25. In arti- 

 ficial lighting, shades acting also as reflect- 

 ors 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 Fin- 

 land, 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 slip- 

 pers 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, especial- 



ly 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 soften- 

 ing, 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 arc 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 "Ap- 

 plication of Microscopic Analysis to dis- 

 covering 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 pecul- 

 iarities of rocks, can recognize fragments, 

 and sometimes even determine the source of 

 the smaller constituents in a composite elas- 

 tic 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, there- 

 fore, probable that a thick and very widely- 



