214 On Adehnamis improved Gumomeier. [Sept. 



them (19) is a support with a hinge, to receive at its upper 

 extremity the crystal to be measured (21); this support slides in 

 a groove (22) in order to bring it nearer to, or further from, the 

 measuring radius, or to withdraw it altogether. The other (20) 

 is a sight-vane, by means of which we can ascertain, after the 

 crystal is fixed on its support, if the edge which separates the 

 two planes, whose incidence is to be measured, be in a perfectly 

 horizontal position. 



To use the goniometer, we place the crystal on the support, 

 in the position just mentioned, then slide the support along the 

 groove, till the measuring radius lies exactly above it, and turn 

 the graduated moveable semicircle, having previously removed 

 the nonius (12) that its motion may be unobstructed. The rest 

 of the operation consists in placing the arm of the measuring 

 radius very accurately on the plane of the crystal, which is 

 opposed to it. This is done by moving the rule (7) to which the 

 semicircle is fixed ; and to do this more easily, and avoid jerks, 

 it is necessary to use both hands, one placed at each extremity 

 of the rule.* If the radius be not placed exactly on the face of 

 the crystal, it is to be gently withdrawn, in order to elevate or 

 lower the moveable semicircle, which is to be done in like man- 

 ner, by using both hands, seizing with one the extremity of the 

 measuring radius, and with the other, the opposite extremity of 

 the moveable circle, till the measuring radius lies so evenly on 

 the face of the crystal, that no day-light can be seen between 

 them when examined with a lens. That done, the moveable 

 nonius (12) is lowered till it meets the stop (23) placed at zero of 

 the moveable semicircle, and fixed by the screw (17). The 

 support is then to be withdrawn , in order to pass the measuring 

 radius on the other side of the crystal, when it is replaced, and 

 the business finished, by repeating on that face of the crystal the 

 same operation as was performed on the other. The value of 

 the angle of incidence of the two planes, in degrees and minutes, 

 is then read ofTon the graduated semicircle and its nonius. 



The instrument is represented in the figure as having finished 

 its operation on one of the faces of the crystal, and with the 

 nonius fixed. 



The use of this goniometer is very easy ; it has the advantage 

 of being fixed, and of not depending for the accuracy of the 

 observations on the manual dexterity of the observer ; nor does 

 it require that perfect brilliancy of surface, in the crystal to be 

 measured, which it is often so dirficult to find. As the measur- 

 ing radius, when once placed on the face of the crystal, remains 



* A regulating screw, with its carriage, was at first used to adjust the motion of the 

 rule, as shown in the figure, at the place where it was fixed ; but it was found not to be 

 necessary, as the adjustment can be made quite as accurately, and much more quietly 

 by hand, provided the rule itself, and the rollers on which it slides, be made perfectly 

 true. 



