356 ON LIGHT. 



two laminae of Mr Herapath's quinine salt, are laid one 

 on the other conformably (or with their axes parallel), 

 the light polarized by one passes freely through the other \ 

 but if the one be turned round on its own plane, on the 

 other, the intensity of the transmittted beam gradually 

 diminishes, until the axes cross at right angles, in which 

 position the combination is quite opake. A similar 

 gradual diminution of light, up to complete extinction, 

 takes place when a ray polarized by reflexion from glass, 

 or in any other manner, is received on such a plate, made 

 to rotate in its own plane. The effects are just what 

 might be expected to happen if a flight of 'flattened arrows 

 were discharged at a grating of parallel wires. Those 

 only whose planes were parallel to the wires would be 

 able to pass, and having passed one such grating, would 

 penetrate any number of others placed conformably be- 

 hind it, but not if placed transversely. This is a simile, 

 not an explanation, but it conveys, though coarsely, to 

 the mind a conception of the distinction between polar- 

 ized and unpolarized light, not to be despised as an aid 

 to the imagination. 



(137.) If a "polarizing frame" of glass plates, such a? 

 above described, be laid down before an open window, 

 and, the eye being held near it so as to embrace a large 

 illuminated area or " field of view," a tourmaline plate be 

 looked through, and turned slowly round, in its own plane 

 before the eye ; a position will be found in which the 

 appearance of a dark cloud comes over the frame, extend- 

 ing over a very considerable visual angle ; the central 

 portion being completely obscure ; and the darkness shad- 



