70 BENJ. PIKE'S, JR., DESCRIPTIVE CATALOGUE. 



Fig. 96. 



11 



a centre, affording the means of examining the whole ver- 

 nier, the connecting arm being long enough to allow the 

 microscope to pass over the whole length of it. 



To the index is attached a clamp to fasten it to the limb, 

 and a tangent-screw, J (in the plate, the clamp is concealed 

 from view), by which the index may be moved any small 

 quantity after it is clamped, to render the contact of the 

 objects observed more perfect than can be done by moving 

 it with the hand alone. The upper end, I, terminates in a 

 circle, across which is fixed the silver-indexed glass, F, over 

 the centre of motion, and perpendicular to the plane of the 

 instrument. To the frame at G is attached a second glass, 

 called the horizon-glass, the lower half of which only is 

 silvered : this must likewise be perpendicular to the plane 

 of the instrument, and in such a position that its plane shall 

 be parallel to the plane of the index-glass, F, when the 

 vernier is set to (or zero) on the limb, B C. A devia- 

 tion from this position constitutes the index error before 

 spoken of. 



