194 



■Sm'alJ errour 

 from parallax. 



May be obvl 

 ateri, 



Accuracy of 

 the instru- 

 ment. 



REFLECTIVE GONIOMETER. 



are parallel to one another (though not in the same plane], 

 glisten at once with the same light, the angle of an irre- 

 gular 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 errour 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 errour may be ren- 

 dered insensible, even in that mode of using the instrument, 

 by due care in placing the crystal ; and when the surfaces 

 are sufficiently smooth to reflect a distinct image of objects, 

 all errour 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, 

 ^nd by turning the small axle may be brought to corres- 

 pond apparently with the bottom of the house (or with some 

 other 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 surfaced may thus 

 be measured, with a degree of precision wliich has not hi- 

 therto been expected in goniometry. 



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

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

 •will probably afford corrections to many former observations. 

 I have already remarked one insfance of a mistake that pre- 

 vails respecting the conjmon carbonate of lime, and I am 

 induced to mention it, because this substance is very likely 



* 1 cannot omit mentioning, that Mr. Soiverby had thought of em- 

 ploying refiecti'sn for this purpose, nearly at the same time as rnyself ; 

 txit did not succeed to his satisfaction, in consequence of an attempt to 

 f.x 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 li- 

 Tnited to the extent of the instrument, and the errour (mtix parallax is 

 manifestly increased. 



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