THE BOOK OF THE LANTERN. 203 



place. The experiments can be varied by altering the 

 position of the magnets, or by using knitting needles which 

 have been just before magnetised at the lecture table. It 

 will be noticed that these curve experiments require no 

 battery power. They are performed with what are known 

 as permanent magnets, in contradistinction to the electro 

 magnet used with the special form of slide shown at 

 fig. 52. 



In describing some of the experiments possible with the 

 optical lantern, I have purposely refrained from detailing 

 any of the splendid effects due to polarised light for these 

 have been already dealt with by my friend Mr. Lewis 

 Wright, in a manner which it would be impossible to 

 improve upon. 1 



1 " Light : a Course of Experimental Optics, chiefly with the 

 Lantern." Macmillan & Co. 



