POLARISED LIGHT. 13 



there are others which are due to convergent light. If light 

 is made to converge on a glass or crystal and to emerge in a 

 divergent form, a variety of effects are produced, not a few 

 of which may be seen by using this piece of tourmaline. 

 You will easily understand if you get a piece of crystal close 

 to your eye you are really examining it by means of a con- 

 vergent and divergent pencil. But if it is placed at the 

 distance of ordinary vision you are using practically parallel 

 light. If it be brought so close to your eye that you use 

 convergent light, you will be able to observe some of the 

 phenomena for which much more elaborate instruments are 

 usually employed. By properly arranging your experiments 

 you may begin with the inexpensive glass plates described 

 before. You may then advance to apparatus a little more 

 elaborate, but even then you may procure a piece or two at 

 a time as you find convenient ; and by degrees build up a 

 polarising apparatus such as I have here. 



I have not time to speak of the applications of polarised 

 light, but 1 have here an instrument which is of commercial 

 value, called a saccharometer, the object of which is to 

 ascertain the amount of saccharine solution present in a fluid. 

 It is based on the principle I last described, namely, this 

 rotary polarisation, as it is called. Besides quartz there are 

 many solutions, particularly those of sugar, which have the 

 power of turning the plane of polarisation of the rays of light, 

 and producing the effects I have described. For a given 

 thickness or length of column the amount of this turning 

 depends on the strength of the solution. Therefore by an 

 accurate measurement of the amount of turning the strength 

 of the solution may be tested. 



Besides the polarisation due to the reflexion of light from 

 furniture, books, leaves and so forth, polarised light is to be 

 found present in a clear sky, and its nature may be tested 

 by any of the instruments described above. 



The full limits of this sky polarisation have not been com- 

 pletely laid down. They were studied in the first instance 

 by Sir David Brewster, and several of his papers, although 

 excellent, have hardly been thoroughly understood ; and 

 indeed the subject is likely soon to undergo fresh ex- 

 amination. Nevertheless you will find it very interesting 

 in walking about on a clear day to examine what parts of 

 the sky show traces of polarisation, and how the plane of 



