A try-square is not always at hand when it is desired to saw a stick, and when it is handy some mechanics prefer to work by “guess” than otherwise. When a bright, straight saw is placed upon a stick or on the edge of a board, the reflection of the stick or board in the saw
is sufficiently well defined to permit of placing the saw so that the reflected image coincides with the object reflected, forming a continuous straight line. If the sawing is done while the image and the stick are in line, the stick will be cut at right angles.
It is obvious that a line may be drawn at right angles to the stick by arranging the saw as shown in Fig 2. If, after forming this line, the saw be placed across the stick so that the line and its reflected image and the stick and its reflected image form a square, with the
reflected image and the stick lying in the same plane, as shown in Fig. 3, the stick may be sawed at an angle of forty-five degrees, provided the saw is held in the same position relative to the stick.
Scientific American – July 26, 1890
– Jeff Burks