Description of a reflective Goniometer, 9 J 



in contact with the surface is necessariW very short, the 

 measures, even when taken with a steady hand, will often 

 deviate too much from the truth to aid us in determining 

 the species to which a suhstance belongs. 



A means of remedying this defect has lately occurred to 

 me, by which in most cases the inclination of surfaces may^ 

 be measured as exactly as is wanted for common purposes; 

 and when the surfaces are sufficiently smooth to reflect a 

 distinct image of distant objects, the position effaces only 

 ^th of an inch in breadth may be determined with as much, 

 precision as those of any larger crystals. 



For this purpose, the ray of light reflected from the sur- 

 face is employed as radius, instead of the surface itself, and 

 accordingly for a radius of -^'^th of an inch, we may sub- 

 stitute either the distance of the eye from the crystal, which 

 would naturally be about twelve or fifteen inches ; or for 

 greater accuracy we may, by a second mode, substitute 

 the distance of objects seen at a hundred or more yards 

 from us. 



The instrument which I use, consists of a circle gradu- 

 ated on its edge, and mounted on a horizontal axle, sup- 

 ported by an upright pillar (Plate II). This axle being 

 perforated, admits the passage of a smaller axle through it, 

 to which any crystal of moderate size may be attached by a 

 piece of wax, with its edge, or intersection of the surfaces,, 

 horizontal and parallel to the axis of motion. 



This position of the crystal is first adjusted, so that by 

 turning the smaller axle, each of the two surfaces, whose 

 inclination is to be measured, will reflect the same light to 

 the eye. 



The circle is then set to zero, or ISO^, by an index at- 

 tached to the pillar that supports it. 



The small axle is then turned till the further surface re- 

 flects the light of a candle, or other definite object, to the 

 eye; and lastly, (the eye being kept steadily in the same 

 place) the circle is turned by its larger axle, till the second 

 surface reflects the same light. This second surface is thus 

 ascertained to be in the same position as the former surface 

 had been. The angle through which the circle has moved, 

 i§ in fact the supplement to the inclination of the surfaces ; 

 but as the graduations on its margin are numbered accord- 

 ingly in an inverted order, the angle is correctly shown by 

 the index, without need of any computation. 



It may here be observed, that it is by no means neces- 

 sary to have a clean uniform fracture for this application of 

 the instrument to the structure of laminated substances ;• for 



since 



