32 OF THE GONIOMETER. 



the first plane, turned the pin x on its axis to bring the 

 reflected image of the bar of the window to coincide 

 accurately with the line below, now move the lower 

 end of that pin laterally, either towards or from the 

 instrument, in order to make the image of the same 

 bar, reflected from the second plane, coincide with the 

 same line below. 



Having assured yourself by looking repeatedly at 

 both planes, that the image of the horizontal bar 

 reflected successively from each, coincides with the 

 same line below, the crystal may be considered as 

 adjusted for measurement. 



Let the 180 on the graduated circle be now brought 

 opposite the o of the vernier at c, by turning the 

 middle plate b ; and while the circle is retained accurately 

 in this position, bring the reflected image of the bar 

 from the Jirst plane to coincide with the line below, 

 by turning the small circular plate a. Now turn the 

 graduated circle from you, by means of the middle 

 plate b, until the image of the bar reflected from the 

 second plane is also observed to coincide with the same 

 line below. In this state of the instrument the ver- 

 nier at c will indicate the degrees and minutes at 

 which the two planes incline to each other. 



