reflective Goniometer. 255 
the graduations on its margin are numbered accordingly in an 
inverted order, the angle is correctly shewn by the index, 
without need of any computation. 
It may here be observed, that it is by no means necessary 
to have a clean uniform fracture for this application of the in- 
strument to the structure of laminated substances ; for since 
all those small portions of a shattered surface, that are parallel 
to one another (though not in the same plane), glisten at once 
with the same light, the angle of an irregular fracture may 
be determined nearly as well, as when the reflecting frag- 
ments are actually in the same plane. 
In this method of taking the measure of an angle, when 
the eye and candle are only ten or twelve inches distant, a 
small error may arise from parallax, if the intersection of the 
planes or edge of the crystal be not accurately in a line with 
the axis of motion ;* but such an error may be rendered in- 
sensible, even in that mode of using the instrument, by due 
care in placing the crystal ; and when the surfaces are suffi- 
ciently smooth to reflect a distinct image of objects, all error 
from the same source may be entirely obviated by another 
method of using it. 
For this purpose, if the eye be brought within about an inch 
of the reflecting surface, the reflected image of some distant 
chimney may be seen inverted beneath its true place, and by 
* I cannot omit mentioning, that Mr. Sow.erby had thought of employing reflec- 
tion for this purpose, nearly at the same time as myself ; but did not succeed to his 
satisfaction, in consequence of an attempt to fix the position of the eye. For when 
the line of sight is determined by a point connected with the apparatus, the radius 
employed is thereby limited to the extent of the instrument, and the error from paral- 
lax is manifestly increased. 
Li 2 
