278 
SIR J. CONROY ON THE AMOUNT OF LIGHT REFLECTED 
Section VI,—Vcdues of the Polarising Angles before and after Repolishing. 
In order to determine the angles of polarisation, a form of apparatus essentially 
similar to that employed by Seebeck (‘ Poggendoree, Annalen,’ vol, 20, 1830, p. 27) 
was used. It consisted of a goniometer with a horizontal circle reading to 20"; the 
slit and lens of the collimator were removed and the observing telescope replaced by 
a tube to one end of which a vertical divided circle was fixed. A Nicol was contained 
in an inner tube, and by means of a vernier the position of its principal section could 
be read on the vertical circle to 5'. The stage of the goniometer and the arm carry¬ 
ing the Nicol were geared together by means of toothed wheels working into a double 
jDinion, the number of teeth in the wheels and pinion being such that on moving the 
arm of the goniometer carrying the Nicol the angular velocity of the tube was twice 
that of the stage. 
A small gas flame was placed close to the end of the collimator tube, the flame 
being surrounded by a blackened metal chimney with a small aperture in it, and the 
glass surface whose angle of polarisation was to be observed clamped to the vertical 
stage with its surface in the prolongation of the vertical axis of the goniometer, the 
stage turned till the image of the flame was seen through the Nicol, and then, by 
means of the pinion (the axis of which was fixed to a sliding piece), the stage and 
Nicol geared together. 
The Nicol having been clamped with its short diagonal horizontal, the arm was 
moved till the light reflected by the glass was reduced to a minimum. 
The end of the collimator tube nearest the lamp was bisected by a vertical thread; 
a pair of cross threads were placed in the inner end, and a diaphragm with a small 
aperture at the eye end, of the Nicol tube ; and in making the observations care was 
taken that the image of the vertical thread should coincide with the point of inter¬ 
section of the two cross threads as seen through the diaphragm. 
The observations were made by moving the Nicol tube alternately towards the 
right and the left. As the image of the flame always remained in the field of view^ 
and the room was completely dark, the angle at wliich the light was reduced to a 
minimum could be observed with a fair amount of accuracy. 
The amount of light reflected by glass at a perpendicular incidence being small, 
and a satisfactory diagonal eyepiece not being available, the position of the stage in 
which the light was incident perpendicularly on the surface of the glass was deter¬ 
mined by an indirect method. 
The axis of the Nicol tube was first, by means of the diaphragm and cross threads, 
brought into the same line as that of the collimator tube, and its position read on 
the divided circle of the goniometer ; the tube was then turned through 90°, the glass 
surface attached to the vertical stage and adjusted, and the stage rotated until the 
imao'e of the sino-le thread ao’ain coincided with the cross threads of the Nicol tube. 
o o o 
