1!)16] on Polarized Light and its Applications to Engineering 575 



is interposed, it selects the components of both waves which are 

 parallel to its principal plane, and their interference causes the glow 

 of colour which jou have already observed. 



Measurement by Colour Effect. 



We can estimate simple stresses by the colours observed. 



If, for example, we take a strip of transparent material, and arrange 

 the optical apparatus so that when the strip is unloaded no light is 

 transmitted, the effect of a moderate tension causes the specimen to 

 appear a greyish white, and as the stress increases, the colour changes 

 by insensible gradations to a lemon-yellow, then to a reddish purple, 

 and, with a very little increase of stress, to a well-defined blue. With 

 a further increase of stress, the scale of colours is approximately re- 

 peated for twice the intensity of stress, and the relation of colour to 

 stress is shown to be that given in the table below. We therefore 

 have a recognizable and easily understood colour scale to mark the 

 intensity of stress in a material. 



For simple tension and compression, the relative retardation of 

 the rays which produces the colour effect, is proportional to the stress 

 and to the thickness of the material. In such cases therefore, the 

 stress intensity may easily be determined by observing the colour 

 bands, bearing in mind that both tension and compression produce 

 similar effects, if changes in the thickness of the material are allowed 

 for. Thus, if we take the case of a transparent beam subjected to a 

 uniform bending moment, a system of colour bands is obtained, dis- 

 tributed as shown in the accompanying experiment, and, by inspec- 

 tion with the aid of Table I, which can be caUbrated in terms 

 of stress, the distribution across the section can be determined 



Table I. 



-Comparative Table of Tension and Compression Stresses 

 Corresponding to a Given Colour. 



Order 



II. 



