crystallized bodies on homogeneous light. 55 
were ascertained not to exceed i° or 2 0 ), must affect the com- 
puted angles of refraction on both sides of the perpendicular 
with equal and opposite errors. The same may be said of 
any error arising from a slight prismaticity of the plate, which, 
however, must have been extremely small, the plate having 
always been rendered parallel by the delicate test of the 
sphaerometer, within a very few divisions.* Consequently, in 
calculating on these data, the mean angle of refraction de- 
termined by the simultaneous use of both observations, (their 
semi-difference being taken for the angle of incidence) may 
be expected to differ from the truth by an extremely minute 
quantity. The third column contains the tint developed in the 
ordinary pencil, and the fourth in the extraordinary. The 
last notices the remarkable points in the system of rings to 
which the tints and angles in the other columns correspond. 
The positions of the poles were determined by interposing a 
red glass between the crystal examined and the reflector used 
to polarise the incident light. The glass used for this pur- 
pose, was of that kind occasionally found in old church win- 
dows, and whose manufacture seems to be numbered among 
arts now forgotten. It transmits almost the whole of the 
red rays, and part of the orange, while it completely stops all 
the more refrangible colours. I have endeavoured in vain to 
procure a specimen, whose limits of transmission are more 
confined. Such are said to exist, though very rare, and in the 
absence of such, the indications of that employed may be 
taken to correspond to the mean red rays. 
* Each equal to the 23809th part of an inch. 
