December 5, 19 12] 



NATURE 



385 



section by Fig. 4. Light from a bank of lamps, A, 

 is diffused by tissue-paper screens, B, and after- 

 wards reflected from a black glass plate, C, set 



ig models of girderi 



Fig. 4.— Cross-section of a polariscope foi 

 and ships. 



at the polarising angle. Quarter-wave plates D 

 and E are arranged to produce a circularly polar- 

 ised field in the object space, F, and for demonstra- 

 tion purposes the analyser is con- 

 structed of thin glass plates, G, 

 while a small Nicol prism is used 

 for quantitative work. This ap- 

 paratus, intended for models of 

 bridge structures and ships, is 

 capable of affording a clear field 

 of view through quarter-wave 

 plates of nearly a yard in length 

 and a foot in depth, but so far no 

 models of this size have been 

 found necessary. 



Polariscopes of a size adapted to 

 show the whole of a model at one 

 time appear to be essential for suc- 

 cessful work in many instances. 

 An example of their use is 

 afforded by a determination of the 

 distribution of stress in a long 

 thin plate, A, Fig. 5, subjected to 



pure shear. 5 A plate of celluloid, 

 3/16 in. thick and 10 in. long, was 

 rigidly clamped at the sides, B, 

 and a maximum pull of about 



three tons was exerted by a cen- 

 trally disposed weight, W , thereby 



affording a nearly pure shear over 



the free portions of the plate. 



The whole of the sheared area was 



visible in the field of view of the Fic. ; 



polariscope, and with the aid of 



a calibrating tension member the distribution 



of shear stress was plotted for different lengths 



5 ".An Optical Determination of the V.irialion of Stress in a Thin 

 Rectangular Plate subjected to Shear." By E. G. Cokcr, Proc. R.S., 



XO. 2249, VOL. 90] 



of plate. The mean shear applied was 800 lb. 

 per square inch in all cases, and the results 

 show some interesting peculiarities. In a 

 long thin plate the shear stress rises slightly 

 m value from the centre to near the ends, and 

 then rapidly falls to a zero value at the extreme 

 edges of the plate. The maxima become more 

 pronounced as the plate is shortened, until a 

 critical length is reached, where the distribution 

 chnnges to one with a central maximum and 

 ultimately becomes parabolic in character with a 

 large increase of intensity, as the final curve shows. 

 Another field of usefulness which suggests itself 

 IS the application of optical science to the design 

 of structural members. If, for example, we take 

 a model eye-bar of a type often used in suspension 

 bridges and the lower chords of pin-connected 

 trusses, we can readily obtain (Fig. 6a) a map 

 of the lines of principal stress for this form, and 

 their general resemblance to those obtained in a 

 hook" at once suggests that across the principal 

 section the stress is very badly distributed. It 

 is apparently very intense at the eye and rapidly 

 decreases until it ultimately changes to com- 

 pression stress at the 'outer end of the section. 

 Experiments now partly completed confirm this 

 view, and they also show that another form (Fig. 

 6b) gives a much better stress distribution whofly 

 tensional across the principal section, as the curves 

 of principal stress indicate. 



Measured shear stress in 

 pounds pr square inch. 

 250 500 750 1000 liSO 



w 



-Distribution of stress i 



I long thin plate subjected to she 



Both forms appear to restrict unduly the lines 

 of stress where the head joins the main member, 



« " The Optical Determination of Slr< 

 1910. 



By E. G. Coker, /V/;7. Mag., 



