12 LECTURES TO SCIENCE TEACHERS. 



rectilinear, or straight line vibrations. I now propose to say 

 a few words upon circular and elliptic polarisation. We may 

 suppose a ray of common light to be due to vibrations of the 

 latter kinds ; but in such a case the direction of motion as 

 well as the position of the axes of the ellipses may be subject 

 to abrupt changes at different parts of the ray ; but when the 

 ray is polarised and the motion in one part of the ray is in 

 the direction of the hands of a watch, say right-handed, it 

 will continue so throughout the entire ray. The well-known 

 crystal quartz has the property, in a certain particular direc- 

 tion, of produciug not only two rays as crystals generally do, 

 but two rays in which the vibrations are circular, one of them 

 being right-handed and the other left-handed. 



We have now gone nearly as far as our time will permit, 

 and I sincerely hope you will repeat some of these ex- 

 periments for yourselves, and that with this view you may 

 take the opportunity of seeing them exhibited here in the 

 evening. But before concluding I would strongly advise any 

 who feel at all interested in the subject to begin with a simple 

 bundle of glass ; the plates should be about the size of your 

 thumb, as thin as you can get, and as numerous as you can 

 work with conveniently. You may use from five to ten or 

 twelve, if you can manage to get them thin and clear enough. 

 Begin your experiments on polarisation by examining reflected 

 light ; and if you are in a room or in a garden take the reflec- 

 tions from the furniture, from the walls, from leaves of trees, 

 or anything of that kind, and you will be interested to find 

 how difficult it is to find any which bear no traces of polari- 

 sation. You may then vary your experience by taking a 

 piece of ordinary mica, such as is used for covering gas 

 burners, split into various degrees of thinness, the thinner 

 the better, place it in the passage of the polarised light, and 

 you will see most vivid gorgeous colours. If you wish to go 

 a little further, you may have an instrument more or less like 

 the one I have here. It consists of a Nicol prism, a double 

 image prism, a plate of quartz, and a piece of crystal called 

 tourmaline. The latter has the same effect of producing a 

 single beam of polarised light and is very convenient, but it 

 has the extreme inconvenience that it is almost impossible to 

 get a plate without colour. 



The phenomena which I have spoken of hitherto are de- 

 pendent upon the properties of rays of parallel light ; but 



