Reflective Goniometer. 77 



tlie degrees upon BB ; and for the better concentration of the visual 

 rays a small knee K, is made to lit upon the line of O on the ver- 

 nier at the angle of vision K. 



The plane II (of maple or any light colored wood) is made to fit 

 the innc}- part of A, both surfaces I and A being in the same plane 

 anteriorly, one inch in depth and seventeen inches in diameter. It 

 revolves upon the iron axis Z, and has a small brass head piece E,' 

 attached to it (and revolving with it) from the center, p, of which is 

 a pin- projecting three fourths of inch from its surface, upon which 

 the crystalline form that we wish to inspect is adjusted and fastened. 

 A semi-circular band of brass BB, three fourths of an inch wide is 

 fastened to the upper edge of II, graduated in degrees correspond- 

 ing to the minutes upon the vernier D. The small rollers a, a, are 

 of iron one fourth of an inch in diameter, secured in the inner plane 

 s, s, of A, for the more easy and true moving of II. And to hold 

 the revolving plane II, immovably upon its arbor Z ; and thereby to 

 prevent BB, from slipping from its place, (after adjusting it by ob- 

 servation,) before we read the angle off; we have a brass clamp 

 C, screwed fast upon the edge of I, and pressing upon a groove in 

 the plane II of A posteriorly by a screw d. (This on the brass in- 

 strument works by a tangent screw.) The semi-circular groovings 

 f,f, are to fit the end of one's fingers in moving the rotary plane I. 

 The whole stands upon a firm support F, M, Q. The legs F, F, 

 (of mahogany two inches square by twelve inches in elevation, i. e. 

 from the top of the stand Q, to the horizontal diameter of AA,) are 

 immovably secured in the mahogany stand or footing Q, which for 

 the better steadying of the instrument has a leaden bottom H, one 

 half of an inch in thickness by twelve inches square, attached to the 

 stand Q, inferiorly, and still further to attain firmness, an iron brace 

 M, one inch by one fourth of an inch, may be fastened to G, with 

 its bent end screwed in the surface of Q. 



To secure the different parts against shrinking, I would not have 

 the given proportions (however clumsy they may appear) lessened. 



Example.-— We first place the instrument upon the table, so that 

 its anterior plane receives the rays as vertically and strongly as may 

 be (reflected through any uninterrupted space, for instance a window) ; 

 then adjust it so that O upon the vernier and O upon the graduated 

 •semi-circle may stand immediately opposite to each other, i. e. 

 K, p, A being in the same horizontal line. Screw the clamp C, fast. 

 And with a bit of wax secure your crystal upon the projection ;?, 



