﻿FresneVs Laws of Reflexion. 



435 



is seen in the same direction as is the undispersed candle-flame 

 by rays which have passed outside the plate. Either spectrum 

 may be used, but the best for the purpose is that formed by 

 the edge nearer the eye. There was now no difficulty in 

 adjusting the index for the yellow ray, and the old effects 

 ought to have manifested themselves ; but they did not. 

 The reflected image showed little deficiency in the yellow, 

 although the incidence was nearly grazing, while at moderate 

 angles it was fairly bright and without colour. This con- 

 siderable departure from Fresnel's laws could only be 

 attributed to a not very thin superficial modification of 

 the glass rendering it optically different from the interior. 



In order to allow of the more easy removal and replace- 

 ment of the plate under examination, an altered arrangement 

 was introduced, in which the aperture at the top of the cell 

 extended over the whole length. The general dimensions 

 being the same as before, the body of the cell was formed by 

 bending round a rectangular piece of tin-plate A (fig. 1) and 

 securing the ends, to which the glass faces B were to be 



C -mentedjby enveloping copper 

 wire. The plate C could then 

 be removed for cleaning or 

 polishing without breaking a 

 joint, In emptying the cell 

 " it is necessary to employ a 

 large funnel, as the liquid 

 pours badly. 



The plate tried behaved much 

 its the que just spoken of. In 

 the reflected light, \yhetlier at 

 moderate angles or nearly gra- 

 zing, the yellow-rgpeen ray uf 

 equal index did not appear to 

 be missing. A line or rather 

 band of polish, by putty- 

 powder applied with the finger, 

 showed a great alteration. 

 Near grazing there was now a dark band in the spectrum of 

 the reflected light as formerly described, and the effect was 

 intensified when the polish affected both faces. In the 

 transmitted light the spectrum was shorn of blue and green, 



the limit coming down a! 



approached — a con- 



sequence of I lie total reflexion of certain rays which then 

 sets in. But at incidences far removed from grazing the 

 place of equal index in the spectrum of the reflected light, 

 showed little weakening. A lew days' standing (alter 



2 G 2 



