154 Dr. Faraday on the Magnetic Affection of Light. 



sponding space on the other end, except that the parts cleared 

 were on the contrary sides of the parallelopiped ; so that each 

 end was furnished with a good plane reflector, but these over- 

 lapped each other (fig. 2). In consequence of this arrange- 

 ment, a ray of light could be transmitted diagonally across 

 the length of the piece of glass; or the ray, after entering at 

 one end, could be reflected two or more times within the glass 

 and then passed out. 



A similar piece of heavy glass was silvered at the two ends 

 and one side of the prism ; and the silvering was then re- 

 moved at the ends for the space of 0*1 of an inch from those 

 edges which were the furthest from the silvered side (fig. 1 ). 

 A ray of light passing in at the unsilvered part of one end 

 with a certain degree of obliquity, could be reflected at the 

 other end, then at the side, and again at the first end, passing 

 thus three times along the glass and 

 finally out at the second end. At Fig. 1. 



other inclinations the ray would pass 

 five, seven, nine, eleven, or a greater 

 number of times along the glass before 

 it issued forth on its course through 

 the air to the eye of the observer. 



Either of these pieces of glass could produce the desired 

 result of repeated reflexions within, but the first form was 

 found most convenient in use. When a strong light was em- 

 ployed, it was not difficult to follow the series of images pro- 

 duced by successive reflexion up to the ninth or tenth image, 

 these corresponding of course to a transit of the ray seven- 

 teen or nineteen times along the substance of the glass. A 

 little change of position of the silvered glass between the Ni- 

 chol's prisms used as the polarising and analysing apparatus, 

 was sufficient to bring any one of these images into view, the 

 glass being at the same time under the full influence of the 

 electro-magnet, or the helix, employed to generate lines of 

 magnetic force. A further advantage is gained if the ends of 

 the piece of glass are not quite parallel to each other, the sides 

 proceeding from the edges where the ray enters and issues 

 forth being in a very slight degree different in length. This 

 arrangement causes the series of reflected images to open out 

 if seen at one end and to close up if seen at the other, and 

 thus the observation of a particular image or the simultaneous 

 comparison of two or more images, is favoured. 



On considering the effect of this arrangement, it is evident 

 that if ABCD represent a trough of solution of sugar, or any 

 other body having the ordinary rotating influence over a po- 

 larized ray, then a ray sent in at D and passing out at A 



