286 
SIR J. CONROY ON THE AlIOUNT OF LIGHT REFLECTED 
Values of p for the Flint Glass. 
Eepolished with putty and examined i 
Original 
Not 
1 
determinations. 
repolished. 
Immediately. 
After an interval. ! 
7 mm. thick . 
•9463 
•9665 
49 
•9508 
•9460 
•9482 
69'5 ,, 
•9461 
•9562 
91-3 „ . . 
•9475 
1 
Mean .... 
•9477 
These results agree with those obtained by the direct measurement of the reflected 
light, and show that the effect of the repollshing is to increase the amount of light 
reflected and to decrease the amount transmitted, and this latter effect must be due 
to some cause other than a more perfect surface having been obtained. 
The polish of the glass plates was examined by holding them close to the apertm^e 
in one of the screens of the photometer, and allowing the beam of light from the 
lamp to pass through the glass, the cross-section of the beam being smaller than the 
surface of the glass, and all other light being carefully excluded. If the surfaces had 
been perfectly polished, they would have been quite invisible, but such was not the 
case, the shadow cast by the edge of the aperture being just visible in all cases. 
Examined in this way, there did not seem to be much, if any, difference between 
the various plates of crown glass, of which two were in their original state and four 
had been repolished, nor between them and the four faces of the flint block, of which 
two had been repolished. 
The surfaces of the crown, and flint glass wedges used for the reflection experiments 
did not appear to be quite so good ; the difference, however, was very slight. The 
7 mm. plate of flint glass was much inferior to all the other pieces, the boundaries of 
the beam of light which passed through it being distinctly visible. When examined 
in a strong light there appeared to be a slight film on the surface; on continued 
rubbing with a wash-leather this diminished, and then the surface of the plate, when 
placed in a beam of light in the dark room, was less visible than before. The 
inferiority of the surfaces of this plate appeared to be due to the films wdiich had 
formed on them, and not to any roughness due to imperfect polishing. 
The truth of Brewster’s law, that the tangent of the polarising angle of a 
substance is numerically equal to its index of refraction, being generally admitted, it 
seemed desirable to ascertain the values of the polarising angles for the different 
plates before and after repolishing. 
Table XXIII. gives the mean results collected from Table XV. The means show 
that the eflect of repolishing was in all cases to increase the polarising angle, a result 
which is in accordance witli those previously obtained. 
