926 



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. 



