368 ON LIGHT. 



them performed in planes at right angles to each other, 

 no such mutual destruction or reinforcement of move- 

 ment can take place. Two movements at riglit angles 

 to each other, communicated at the same instant to the 

 same material molecule, combine, in virtue of the me- 

 chanical principle of the composition of motions, to pro- 

 duce a movement intermediate in direction ; and can in 

 no case destroy each other. 



(147.) It follows from this, that if such be really the 

 nature of the luminous vibrations and such the true ex- 

 planation of the phasnomenon 'of polarization interfer- 

 ence can only take place between rays polarized in the 

 same plane such complete interference at least as shall 

 result in the extinction of both, in the manner above de- 

 scribed. This conclusion is, happily, capable of being 

 brought to the test of experiment, and the result is found 

 to be in exact accordance with the a priori reasoning. 

 The experiment is simple ancj direct. Let two small 

 holes, or, better, parallel slits very near each other in a 

 thin opake screen, be placed between the eye and a 

 very minute and brilliant point of light ; and viewed 

 through a lens, as described in a former paragraph ; so 

 as to see the diffractive fringes. Now over the holes or 

 slits let two plates of tourmaline of precisely equal thick- 

 ness and in every respect similar be applied (to secure 

 which conditions, the two halves of a single plate worked 

 to exact parallelism and cut across, may be used). Then 

 if the axes of these plates be parallel, in which case the 

 light passing through both the apertures will be similarly 

 polarized, the fringes will continue to be seen. If one 



