REFLECTIVE GONIOMETER. 65 



"Upon its edge, and disposed vertically upon a moveable, horizontal 

 axis ik, which is supported by the braces ran and mo, inserted into 

 the circular horizontal foot gh. This circle is furnished with a ver- 

 nier q, which is attached to the support ran. The axle ik is hollow, 

 and traversed by a second one if: both may be revolved upon them- 

 selves by means of the circular wheels v and s, with this difference, 

 that the lesser wheel turns only the inferior axis, the outer one, to- 

 gether with the circle, remaining stationary; whereas, the larger 

 wheel turns all at once, the exterior axle, the circle which is adapt- 

 ed to it, and the interior axis. 



The interior axis is prolonged from/, at first by a semi-circular 

 projection, consisting of two pieces and joined together by a rivet 

 at d, so as to allow of motion in the portion Id. Its extremity I is 

 pierced and traversed by a round stem ep> capable of being moved 

 up and down, and at the same time of being turned circularly by 

 the little wheel u. The extremity;? is slit so as to receive a small 

 copper plate c. 



The instrument is used as follows. It is first placed on a small 

 pyramidal stand, and the stand on a steady table. The stand should 

 be of su<5h a height above the table as to permit the experimenter to 

 sit with both elbows upon the table, while his eye shall not be ele- 

 vated above the axis ik. Tha stand should be firmly attached to the 

 table, and the goniometer to the stand. The table is now placed be- 

 fore a common flat window, at the distance of from six to twelve feet, 

 in such a manner, that the vertical wheel or circle shall be as nearly 

 as possible perpendicular to it. A black line is drawn on the wain- 

 scot, between the window and the floor, perfectly parallel with the 

 horizontal bars of the window. The crystal to be measured is at- 

 tached, by means of a piece of wax, to the plate e, or (dispensing 

 with the use of the plate c) to a piece of wax half an inch long, 

 extending horizontally in the direction of the axis ik. In attaching 

 the crystal, we endeavor to adjust it, so that the edge formed by tho 

 meeting of the two planes whose inclination is sought, shall coincide 

 with an imaginary line passing through the axis ik. It is impossible 

 to effect this simply by inspection : the following steps are therefore 

 taken to ensure the accuracy of this adjustment. One of the planes 

 forming the angle sought is brought uppermost, and so as to be as 

 nearly parallel to the table as possible, when the eye is placed so 

 near that the lower lid nearly touches it ; in this situation the crysr 

 tal is not seen, but we observe distinctly the images of objects re^ 

 fleeted from the plane under the eye, and by giving a slight motion 



