of Achromatic Object-Glasses. 3 
as shewn in the figure : m m are two arms turning on one com- 
mon centre, coinciding with that of the circle, each being fur- 
nished at its extremity with a disc, having an adjustable sight 
pierced with a fine hole: ef is a brass plate adjustable by the 
tangent-screw, seen in Fig. 2., and which plate carries at the 
upper part a square frame fixed at right angles to it. This 
square frame is counter-sunk on the inside, so as to receive a 
parallel plate of glass, on which the prism is placed, for observa- 
tion, as seen in both figures. 
The nature of this frame will be better understood by the 
perspective view of it, shewn in Fig. 3. Fig. 4. is a brass-plate 
ground parallel, and made to slide accurately over the frame, in 
such a way as to bring the straight chamferred edge a h exactly 
opposite to the centre of the graduated circle, — and the prism, 
when placed on the glass-plate, is brought exactly in contact 
with this line or edge. The tangent-screw mentioned above, 
serves to adjust the frame upwards or downwards, till the edge 
a b of the plate is opposite the centre, as above stated. 
3. To measure the Angle of the Prism . 
For this, it is best to use the parallel glass, blackened at the 
back, or to keep one glass for this purpose: lay this in the 
frame, and, by means of a short spirit-level laid upon it, adjust 
the instrument by the screws in the stand, till it is perfectly ho- 
rizontal; then slip on the brass-plate, which ought also to be 
blackened, to prevent any confusion of reflected light. Now, 
bring both arms of the instrument above the horizontal line or 
zero, and set them both by means of the verniers to the same 
angle ; as, for example, 40° or 50°, &c. Then looking through 
one of the small holes in the sights, the reflection of the other 
ought to be seen bisected by the edge of the brass-plate, which 
will shew the instrument to be correct ; and if this should not 
happen, it must be brought to do so by adjusting the sights ac- 
cordingly. This being done, lay on the prism, placing its sharp 
edge gently against the edge of the plate above mentioned, and 
then, while one of the sights remain fixed, move about the other, 
till the reflection of the small hole in the former is seen bisected 
by the straight edge as before, and then half the difference in 
the two readings will be the angle sought. This operation, which 
