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1176 HANDBOOK OF PROJECTION FOR
with one of its edges perfectly straight. That last is important. From one foot to eighteen inches in length will be ample and convenient. To its straight edge attach a wooden strip, either by small nails or screws, permitting it to extend above the surface of the board by at least one-eighth of an inch. The end of such a board is shown in Fig. 429. Next measure the length of one of the needles you are using and, using a straightedge, draw a pencil line on the surface of the board a distance from the backing strip equal to the length of the needle you measured. The board is now done, except that you might nail a strip at either end to prevent the needles from rolling off.
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Figure 429.
To examine needles, lay them in a row, butts to the backing strip. The line on face of board will help detect variations in length. If you have a machinists' scale or other piece of metal six inches to a foot long, you may detect and remove undersize needles merely by pressing the bar down on the row of needles, at the same time tipping the board. The small needles will of course slip out, and may be placed by themselves.
However, this latter method of separating the small diameter needles must be used carefully. Wood has a certain amount of resiliency and if you press too hard the thicker needles will sink into it enough to permit of the bar clamping the smaller ones enough to hold them. As a matter of fact the theatre will do well to provide a cast iron needle testing block. Any machine shop possessed of a planer would make one for a small sum.