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CH. IX] HOME-MADE PROJECTION APPARATUS 291
This will make a track along which the blocks will move freely. If both tracks were firmly fixed the blocks would have to be constructed with extreme precision or the blocks would bind. They would also bind if the tracks were not perfectly parallel at all points. The loose track gives slightly and thus compensates for any little irregularity of the track or apparatus block.
§ 428. Apparatus blocks.— These are shown in figures 158, 159. They must be sufficiently heavy so that the various pieces of apparatus they carry will be steady ; and finally the sockets for receiving the stems of the apparatus must be on the blocks in a position so that the parts like the stage and the microscope can be brought sufficiently close together.
Size and weight of the different blocks for the apparatus figured (fig. 158, 159):
1. Arc lamp block. 12^ x 12^" cm. (5x5 in.) ; weight i kilo. (2 Ibs.), (fig. 158).
2. Condenser block. 12^x10 cm. (5x4^.); weight 2 kilos. (4^ Ibs.), (fig. 1 58).
3. Stage block, i2>^ x 6 cm. (5x2 in.) ; weight, i kilo. (2 Ibs.), (ng. 158).
4. Microscope block, 12^x10 cm. (5x4 in.); weight, 2-3 kilos. (4-6 Ibs.), (fig. 158).
5. Block for lantern-slide carrier, 12^x6 cm. (5x2 in.); weight # kilo, (i lb.), (fig. 158).
6. Block for lantern objective, or a photographic objective, (fig. 158), 12^2 x 10 cm. (5x4 in.); weight, 2 kilos. (4^2 Ibs).
7. Block for horizontal microscope, 17x12^ cm. (7x5 in.), weight 2>£ kilos. (5^ Ibs.), (fig. 145).
§ 429. Construction of the apparatus blocks. — If one has the
facilities of a machine shop and foundry at his disposal these apparatus blocks may be made of cast iron, smoothed and grooved on a planer. In like manner the lathe bed with V's can be made (fig. 134). Lacking these facilities one can prepare blocks of wood which will answer almost perfectly as follows: Select some fine grained board 2 cm. to 2.5 cm. thick (% to i in.), and cut it into