66 TERMINOLOGY. 



to the lesser wheel, if necessary, we obtain reflections of some of the 

 horizontal bars of the window. One of these, which is capable of 

 being recognized, is selected to be employed in the experiment, and 

 brought down by the turning of the axle so as to coincide with the 

 black line upon the wainscot, as seen directly with the eye. It is 

 seldom or never the case that the crystal, as first adjusted, affords 

 this coincidence, the line visible in the face of the crystal almost 

 always forming an angle more or less acute with the line on the 

 wainscot seen directly. In order to effect this coincidence, we com- 

 municate to the crystal a variety of slight movements, by means of 

 the hinge at d, or of the little wheel e; by one or both of these mo- 

 tions the adjustment is brought about. This being done, the axis is 

 turned and the same arrangement effected with regard to the other 

 plane. Sometimes it happens, that in adjusting the second plane 

 we disturb the first: a little attention will, however, enable us to 

 fix them both in the requisite position. Both reflections being pre- 

 cisely arranged with regard to the black line, it is next requisite to 

 observe that the line at 180 upon the circle forms a line with that at 

 upon the vernier, at the same time that the reflection of the win- 

 dow bar is seen along the black line. Now we have only to turn 

 the exterior axis until the image of the bar reflected from the sec- 

 ond plane is in like manner observed to coincide with the same line 

 below. In this state of the instrument, the vernier at c will indi- 

 cate the degrees and minutes at which the two planes incline to 

 each other. Suppose the angle to be 105 5'. In this case, we 

 shall find 105 upon the circle, as the nearest number which touches 

 the on the vernier; still the line belonging to 105 upon the rim of 

 the circle has perceptibly passed that corresponding to on the ver- 

 nier. In order to ascertain the precise measure of this distance in 

 minutes, we notice which line on the vernier cuts, or forms but one 

 line with another" line on the principal circle, In the present case, 

 it will be the line marked 5 on the vernier. 



The method above described cannot, however, be followed to the 

 best advantage by persons who are short sighted. Glasses cannot 

 be used in these experiments, and the sight of such persons will 

 not in general, allow of their observing the black line with sufficient 

 distinctness without them. By the following arrangement, how- 

 ever, near sighted persons may use the reflective goniometer with 

 the utmost accuracy. Let a cyphering slate be set up on edge 

 by means of two cross pieces of wood, so that it shall stand perpen- 

 dicularly and steadily upon the table between the goniometer and 



