** An Optical Illusion [130]

When looking at the accompanying sketch you will say that the letters are alternately inclined to the right and left.  They are not so and can be proved by measuring the distance of the top and bottom of any vertical strokes from the edge of the entire block.  They will be found to be exactly the same distance.  Or take any of the horizontal strokes of the four letters and see how far their extremities are from the top and bottom of the entire block.  It will be found that a line joining the extremities of the strokes are strictly parallel to the top or bottom and that they are not on a slant at all.  It is the slant of the numerous short lines that go to make up the letter as a whole that deceives the eye.

[Illustration:  Illusion]

** Home-Made Micrometer [130]

It often becomes necessary to find the thickness of material so thin, or inconvenient to measure, that a rule or other measuring device will not serve the purpose.  A simple, fairly accurate, and easily made apparatus of the micrometer form may be constructed as shown by the accompanying sketch.  Secure a common iron or brass bolt about 1/4-in. in diameter and about 2-1/2 in. long, with as fine a thread as possible, and the thread cut to within a short distance of the head of the bolt.  The head of the bolts should have a slot cut for the use of a screwdriver.  Clamp together two blocks of wood with square corners which are about 1 in. wide, 3/4 in. thick and 2-1/2 in. long and fasten them together with small pieces nailed across the ends.  The width of the blocks will then be about 2 in.  Bore a 1/4-in. hole through the center of the blocks in the 2 in. direction.  Remove the clamp and set the nut into one of the blocks, so that the hole will be continuous with the hole in the wood.  Cut out a piece from the block combination, leaving it shaped like a bench, and glue the bottoms of the legs to a piece of thin board about 2-1/2 in. square for a support.

Solder one end of a stiff wire that is about 2 in. long to the head of the bolt at right angles to the shaft, and fix a disc of heavy pasteboard with a radius equal to the length of the wire, and with its circumference graduated into equal spaces, to serve in measuring revolutions of the end of the wire, to the top of the bench.  Put the bolt in the hole, screwing it through the nut,

[Illustration:  Micrometer]

and the construction is complete.  The base is improved for the measuring work by fastening a small piece of wood on the board between the legs of the bench.  A small piece of metal is glued on this piece of wood at the point where the bolt meets it.

Find the number of threads of the screw to the inch by placing the bolt on a measuring rule, and counting the threads in an inch of its length.  The bolt in making one revolution will descend a distance equal to the distance between the threads.