Page:Popular Science Monthly Volume 88.djvu/954

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��Popular Science Monthly

���How to Make a Polariscope to be Used with a Microscope

MANY micro- scopic objects that appear unin- teresting and de- void of structure when examined in the ordinary way, develop surprising beauty of form and color when \iewed b>' polarized light. This applies to the majority of crystals and rock sections, as well as many vegetable sections contain- ing minute crystals embedded in the tissues. A beam of light may be polarized by passing through a specially cut prism of Iceland spar, or more cheaply, by using a bundle of glass plates inclined at a certain angle. The polarized beam is allowed to pass through a transparent object and afterward through a second prism or bundle of glass. The polariscope will therefore consist of two parts, one placed beneath the stage and called the polarizer; the other somewhere above the object, and called the analyzer. The best position for the analyzer is usually considered to be just inside the body- tube of the microscope, immediately above the objective. A low-power ob- jective measuring about i in. is best when working with polarized light.

The accompanying illustration shows the details of the polarizer in section. The several parts may be mounted in a brass tube A. Any tube that happens to be handy will do, but it must make a nice fit in the understage fitting of the microscope, so as to be capable of rotation without danger of falling out. A paper tube can be used as a substitute, though of course it will be less durable than metal. If paper is used the tube should be made by coating one side of a strip of paper with thin glue or good strong paste and winding tightly around a rod or tube of suitable size, care being taken to prevent the formation of wrinkles. If several layers of paper are wound on, the tube will be hard and strong when dry. One end must be closed with a cap B, perforated in the center with a hole 3^ in. in diameter and projecting sufhciently beyond the tube to afford a convenient grip.

��Two pieces of cork C, C, must fit neatly in the tube. Each of these must be cut as shown in the illustration, the slanting sides forming an angle of 57 degrees with the side of the tube. Further, both corks must be perforated with a hole of the same size as that made in the cap B. The holes should be carefully made with a cork-borer so that they will be continuous when the parts are assembled, and parallel to the axis of the tube. They should be black- ened inside with photographic dead- black, or else lined with black paper having a dull surface. After fixing one cork by means of fish glue, a number of thin microscopic cover-glasses D should be dropped in, each of which must first be cleaned thoroughly with tissue paper or chamois leather. About 18 will be sufficient. They are best handled with a pair of small pointed forceps. The second cork can then be inserted, a gentle pressure being applied to keep the thin glass plates from moving and so rubbing dust off the corks.

The analyzer is merely a replica of the polarizer, but small enough to go inside the body-tube of the microscope. In this case it will not be necessary to have a cap at the end of the tube, since, if the polarizer rotates, the analyzer does not need to move.

A polariscope made in this way is in- expensive and the results, though some- what inferior to those obtained by the use of Iceland spar prisms, will repay the trouble of preparation. One or two selenite films should be purchased, mounted on microscope slides of the ordinary size, 3 ins. by i in., and placed immediately beneath the object. By this means, the range of color is greatly increased. — H. T. Gray.

To Stop a Lathe Quickly

WHEN polishing or turning small di- ameters in the lathe it is usual to speed the lathe up to its limit. This is all right, but in stopping it is the custom to throw the belt shifter quickly, which often causes the reverse clutch to be en- gaged ; and if it happens suddenly the result is that the belt comes off. This trouble can be easily prevented by plac- ing a collar on the shipper rod which will prevent the reverse clutch from engaging.

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