2 LECTURES TO SCIENCE TEACHERS. 



generally to possess. An instance of this is furnished by 

 the familiar process of reflexion. When a ray of light 

 falls upon a smooth or polished surface it is usually reflected, 

 whatever be the angle of incidence upon the surface. And 

 the same is true, even if the ray should undergo reflexion at 

 a second, or a. third, or at any number of surfaces. There 

 are, however, particular substances which, although presenting 

 polished surfaces, do at certain angles of incidence fail to 

 give riso to a reflected ray, when after one reflexion the ray 

 has fallen upon a second surface. Again, there are transparent 

 Substances whrch , at certain angles of incidence fail to 

 transmit a ray of light which has previously undergone 

 reflexion or refraction under suitable cpnditions. Again, 

 there are dispositions of perfectly colourless transparent 

 crystals through which, if ordinary light be transmitted, 

 splendid effects of colour are produced, and this in a way 

 quite distinct from the prismatic dispersion explained yester- 

 day by Professor Stokes. But in all cases where, contrary to 

 expectation, either the reflected or the refracted ray is absent, 

 or where colour is so produced, it will be noticed that the 

 peculiarity occurs, not at first a reflexion or refraction, nor 

 when a single piece of crystal is used, but when the light, 

 having already undergone one of these processes, falls upon 

 a second reflector or refracting substance. The light must in 

 fact have first undergone some modification, and the effect of 

 the modification is brought to evidence at a second process. 

 This modification, which takes place in various forms, and 

 which may be effected in various ways, is called Polarisation 

 and the process by which light is examined, whether it be 

 polarised or not, is called Analysation. Similarly, the instru- 

 ment, of whatever kind, used for the first process, is called 

 the Polarizer ; and that used for the second the Analyser. 



Let us now come to closer quarters with the subject. If 

 I take a flat plate of ordinary glass and interpose it in the 

 path of a beam of light, part of that, light will be reflected 

 from the surface, and part will be transmitted through the 

 plate. And this will be the case at whatever angle the light 

 falls upon the glass. And if either the reflected or the 

 refracted beam be received upon a second plate, the same 

 results will in general ensue. If, however, the beam of 

 light being in a definite direction, say horizontal, the first 

 plate of glass be inclined at about one third of a right angle 



