REFLECTIVE GONIOMETER. ]g^ 



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 of faces only 

 ■^V of an inch in breadth may be determined with as much 

 precision as those of any U>rger crystals. 



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

 face is employed as radius, instead of the surface itself, and ^"g^^^^aed ra. 

 accordingly for a radius of jV of an inch, we may substitute 

 cither 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 iustrumeut which I use consists of a circle graduated Instrument de- 

 on its edge, and mounted on a horizontal axle, supported ^""'^^*^' 

 by an upright pillar (Plate V.) 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 Method of 

 turning the smaller axle, each of the two surfaces, whose "^'"S ''• 

 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 pi 'ar that supports it. 



The small axle is then turned till the farther 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 ascer- 

 tained to be in the same position as the former surface had. 

 been. The angle through which the circle has fiioved is in 

 fact the supplement to the inclination of the surfaces; but 

 as the graduations on its margin are numbered accordingly 

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

 dex, without need of any computation. 



It may here be observed, that it is by no means necessary Angle of an 

 to have a clean uniform fracture for this application of the irregnlar fra»- 

 instrument to the structure of laminated substances; for l,"ea5u^rcd by 

 since all those small portions of a shattered surface, that \u 

 VOL.XXV.— Mar. 1810. O are 



