p6 Description of a reflective Gmiiometer, 



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 irregfu- 

 lar fracture may be determined nearly as well, as when the 

 reflecting fragments 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 ren- 

 dered insensible, even in that mode of using the instru- 

 ment, by due care in placing the crystal ; and when the 

 surfaces are sufficiently 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 turning the small axle may be brought to correspond 

 apparently with the bottom of the house (or with some 

 otljer distant horizontal line). In this position the surface 

 accurately bisects the angle, which the height of that house 

 subtends at the eye (or rather at the reflecting surface) ; then, 

 by turning the whole circle and crystal together, the other 

 surface, however small, may be brought exactly into the 

 same position ; and the angle of the surfaces may thus be 

 measured, with a degree of precision which has not hitherto 

 been expected in goniometry. 



The accuriicy, indeed, of this instrument is such, that a 

 circle of moderate dimensions, with a vernier adapted to it, 

 will probably aiford corrections to many former observa- 

 tions. J have, already remarked one instance of a mistake 

 that prevails respecting the common carbonate of lime, 

 and I am induced to mention it, because this substance is 

 very likely to be employed as a test of the correctness of 

 such a goniometer, by any one who is not convinced of it» 

 accuracy from a distinct conception of the principles of its 

 construction. 



The inclination of the surfaces of a primitive crystal of 

 carbonate of lime is stated, with great appearance of pre- 



* I cannot omit mentioning, that Mr. Sov/erby had thought of employin;^ 

 reflection for ihis purpose, nearly at the same lime as myself; but did noE 

 j*iicceed to his satisfaction, in conseijuence ot an attempt to fix the positiou 

 of the eye. For when the line of siglit u determined by a point connected 

 ■with the apparatus, the radius employed is thereby limited to the extent of 

 xhd insuumeut, and the error from parallax is manifestly increased. 



cision. 



