OF THE GONIOMETER. 31 



required, and with the edge at which those planes 

 meet, the farthest from you, and parallel to the win- 

 dow in your front. 



Fi- 69. 



Attach a portion of wax about the size of d, to one 

 side of a small brass plate e, fig. 59 lay the plate on 

 the table with the edge/ parallel to the window, the 

 side to which the wax is attached being uppermost, 

 and press the end of the wax against the crystal until 

 it adheres ; then lift the plate with its attached crys- 

 tal, and place it in the slit of the pin jr, with that 

 side uppermost which rested on the table. 



Bring the eye now so near the crystal, as, without 

 perceiving the crystal itself, to permit your observing 

 distinctly the images of objects reflected from its 

 planes; and raise or lower that end of the pin ,r 

 which has the small circular plate affixed to it, until 

 one of the horizontal upper bars of the window is 

 seen reflected from the upper orjirst plane of the crys- 

 tal, which corresponds with plane , fig. 55 and 56, 

 and until the image of the bar is brought nearly to 

 coincide with some line below the window, as the 

 edge of the skirting board where it joins the floor. 



Turn the pin x on its own axis, if necessary, until 

 the reflected image of the bar of the window coincides 

 accurately with the observed line below the window. 



Tutn now the small circular plate a on its axis, 

 and from you, until you observe the same bar of the 

 window reflected from the second plane of the crystal 

 corresponding with plane &, fig. 55 & 56, and nearly 

 coincident wiili the line below ; and having, in adjusting 



