3 o6 SCIENTIFIC APPARATUS. 



parameters, gave the distances at which any and every other face 

 of the crystal would, if continued, meet the axes. Here, therefore, 

 in asserting the integral character of the numbers representing 

 these coefficients (or indices) belonging to every face of the crystal 

 was the fundamental law of crystallography laid down on a purely 

 geometrical basis. Contemporaneously chemistry was asserting 

 an analogous law in the recognition of the integral character and 

 general simplicity of the numbers by which the chemical equivalents 

 are multiplied in combination and in both sciences the venerable 

 axiom, Naturb nihilfit per saltum, was belied by what appeared in 

 each case to be most simply explicable by the hypothesis of the 

 atomic theory ; more strictly pernaps of a molecular theory in the 

 case of the constitution of crystallized matter. 



With the progress of crystallographic geometry in the hands of 

 Weiss, and of the great mineralogist Mohs, a more exact instru- 

 mental method for determining the angles of crystals became 

 necessary, and this was supplied by the reflective goniometer of 

 Wollaston, the original instrument which, under various and great 

 modifications, is essentially the goniometer of to-day. Wollaston's 

 goniometer consists of a graduated circle on which minutes may 

 "be read by aid of a vernier ; its centre being penetrated by a double 

 axle, one axle being capable of revolving within the other, so that 

 they can be moved independently or together, the graduated circle 

 being attached to and moving with the outer or hollow axle. In the 

 original instrument the graduated circle moved in a vertical plane : 

 in the form of it employed by Professor W. H. Miller this plane is 

 horizontal. In either case the crystal is supported at the extremity 

 of the prolonged inner axle, and the modes employed for its attach- 

 ment, support, and adjustment have been various. The measure- 

 ment of the angle at which two faces of a crystal meet in an edge 

 is effected by bringing into coincidence two signals, one a bright 

 one preferably a ray of sunlight, to which direction is given by a 

 heliostat the other a dull one simultaneously seen, the former by 

 reflection from each successive face forming the edge, the latter 



