4 Mr Barlow On the Practical Construction 
is very simple, may be repeated at several angles, and the mean 
result taken for the angle of the prism. 
4. The following examples will sufficiently illustrate this ope- 
ration. 
Flint Prism , No. 1. 
Fixed sight. 
Moveable 
sight. 
Difference. 
^ Difference 
or Angle. 
40° O' 
89° 38' 
49° 38' 
24° 49' 
30 0 
79 36 
49 36 
24 48 
35 0 
84 40 
49 40 
24 50 
36 0 
85 38 
49 38 
24 49 
25 0 
74 38 
49 38 
24 49 
Mean angle, 
24 49 
Plate Prism , No. 1. 
Fixed sight. 
Moveable 
sight. 
Difference. 
^ Difference 
or Angle. 
40° O' 
89° 42' 
49° 42' 
24° 51' 
35 0 
84 40 
49 40 
24 50 
30 0 
79 44 
49 44 
24 52 
25 0 
74 44 
49 44 
24 52 
20 0 
69 40 
49 40 
24 50 
Mean angle, 
24 51 
The principle of this deduction is too obvious to call for any 
farther remark, than merely to state, that it is founded on the 
known law, that the angle of incidence is equal to the angle of 
reflection. 
5. Observations for determining the Index of Refraction. 
It is a known principle in optics, that, in the passage of light 
out of one medium into another, as, for example, from glass into 
air, the sines of the angles of incidence and refraction are to each 
other in a constant ratio ; and this ratio is what is called the in- 
dex of refraction. 
In order to determine the data requisite for ascertaining this 
index, we must proceed as below. 
Having adjusted the instrument as before, place in the frame 
the clear parallel plate of glass, instead of the blackened one used 
in the last case, and apply the blackened brass plate as before ; 
bring also the edge of the prism in contact with the edge of the 
plate, as described in the last observations. 
