612 
MR. R. T. GLAZEBROOK ON THE REFRACTION OF PLANE 
to coincide with the end of the needle point in the focus of the telescope. The tele¬ 
scope was then turned to view the images reflected from the faces of the prism, and 
the levelling screws of the prism adjusted until the same coincidence was established. 
Now observation showed that when P and Q were level in this manner R x and R 3 
were both a very little too low. 
The vertical angular distance between the end of the needle and the mark across 
the slit, seen by reflection from Ft, or R 3 , could be estimated by setting the needle on 
the slit, reading the vernier of the telescope and then moving the telescope with the 
needle until the horizontal distance between it and the slit appeared about equal to 
the vertical distance between the end of the needle and the mark. 
This horizontal distance is found at once by again reading the vernier of the 
telescope. 
I found as the mean of several closely concordant observations that when the angle 
of incidence of the light on the face R L was 20° 58' the image formed by reflexion was 
10' too low, and the same exactly was the case with the image formed by reflexion at 
the same angle from R 3 . 
Let L O (fig. 5) be the direction of the incident light from the mark on the slit, 
0 T the reflected ray, O R L the normal to I\ L . 
L II, T is a great circle, let L N K be the principal plane of the prism, T K and 
R : N being perpendicular to L N K. 
lb 1 =^=b 1 t 
Let the angle TLK=y, R 1 N=££. 
The experiment has shown that TK = 10'. 
From the triangles T L K, R : L N 
sin x— sin y sin <f> 
sin 10'= sin y sin 2 <j> 
and 
<fr=.20° 58' 
whence 
x=6' 
Thus R 3 is 6' below the great circle L K, and so also is R 3 . 
