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 con- 

 structed 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 appara- 

 tus 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. (5 x 5 in.) ; weight i kilo. 

 (albs.), (fig- 158). 



2. Condenser block. 12^ x 10 cm. (5 x 4 in.) ; weight 2 kilos. 

 ( 4 # Ibs.), (fig. 158). 



3. Stage block, 12^ x 6 cm. (5 x 2 in.) ; weight, i kilo. (2 Ibs.), 



(fig- 158). 



4. Microscope block, 12^2x10 cm. (5x4 in.); weight, 2-3 

 kilos. (4-6 Ibs.), (fig. 158). 



5. Block for lantern-slide carrier, 12^2x6 cm. (5 x 2 in.); 

 weight Y 2 kilo, (i lb.), (fig- 158). 



6. Block for lantern objective, or a photographic objective, 

 (fig. 158), i2>^ x 10 cm. (5x4 in.); weight, 2 kilos. (4^ Ibs). 



7. Block for horizontal microscope, 17 x 12^ cm. (7x5 in.), 

 weight 2^/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 ( 7 /$ to i in.), and cut it into 



