160 Dr. Brewster on the communication of double 
Scholium. 
The experiments now described, furnish us with a method 
of rendering visible, and even of measuring the mechanical 
changes which take place during the compression, dilatation, 
or bending of transparent bodies. The tints produced by 
polarised light are correct measures of the compressing and 
dilating forces, and by employing transparent gums, of diffe- 
rent elasticities, we may ascertain the changes which take 
place in bodies, before they are either broken or crushed. 
The subject, therefore, of the strength of materials, and the 
cohesion of solids, will derive new lights from the principles 
already established. 
There is one practical application of these views which is 
particularly deserving of notice. In order to observe the 
manner in which stone arches yield to a superincumbent pres- 
sure, Dr. Robison executed several models in chalk, and 
deduced many general laws relative to the internal forces by 
which they were crushed. If the arch stones of models are 
made of glass, or any other simply refracting substance, such 
as gum copal, &c. the intensity and direction of all the forces 
which are excited by a superincumbent load in different parts 
of the arch, will be rendered visible by exposing the model to 
polarised light. If different degrees of roughness are given 
to the touching surfaces of the glass voussoirs, the results 
may be observed for any degree of friction at the joints. The 
intensity and direction of the compressing and dilating forces 
which are excited in loaded framings of carpentry, may be 
rendered visible in a similar manner. 
