576 Professor Ernest G. Coker [Feb. l-S^ 



approximately, as shown in the diagram. It is seen that the stress 

 varies uniformly from a maximum compression at the under side to 

 nothing near the centre, and it then changes sign and increases 

 uniformly to a maximum tension stress at the upper side of the beam, 

 in exactly the same manner as occurs in a metal beam subjected to the 

 same kind of loading. 



This case and others which have been examined afford instances 

 where the results of optical experiments can be compared, not only 

 with mechanical measurements of strain, but also with the theory of 

 the distribution of stress in materials ; and the experimental de- 

 terminations for a transparent material show a very good agreement 

 with strain measurements and with the precise theory. Wc can, 

 therefore, feel very confident that in more complicated cases the 

 stresses in a transparent model are similar to those in a metal. For 

 example, a beam with a notch cut in it may be taken (as shown), and, 

 as might be expected, the effect of the notch is seen to increase the 

 stress in the material very considerably. The distribution is now 

 much more complicated than it is in a simple beam ; the neutral axis 

 has moved towards the notch, while the colour effects show that the 

 maximum stress is at least twice as great as that in a beam without a 

 notch. 



To apply this kind of experimental work to cases of practical 

 importance, it is evidently desirable that the material used should 

 possess similar physical characteristics to those possessed by steel, 

 iron, and other materials in general use by engineers, and in this 

 respect glass has very many desirable qualities. Unfortunately, 

 however, in order to produce colour effects in glass specimens, it is 

 necessary that the material should be very thick, and the forces to 

 which the glass is subjected very great — in fact, very near the 

 breaking-point of the material. Glass is, moreover, an extremely 

 difficult material to shape, and specimens can only be produced by 

 laborious processes. 



The extensive commercial value of nitro-cellulose preparations in 

 recent years, however, affords a material which has nearly all the 

 desirable features of glass, and, in addition, some other characteristics 

 which make it superior in many respects for experimental work. 

 This material may be obtained in large sheets of moderate thickness, 

 entirely free from stress except at the edges. It is not so transparent 

 as glass, but it has the very important advantage that specimens may 

 be cut from the sheet quite readily with ordinary wood and metal- 

 working tools, and with reasonable care the contours of the specimen 

 show no signs of residual stress after shaping. 



An example of its value is afforded by the accompanying model 

 of a roof -truss in which the riveted joints havs been accurately con- 

 structed from the drawings supplied, while another is supplied by 

 the cog-wheels in gear, which have been machined in exactly the 

 same manner as their metal counterparts. 



