GONIOMETER AND MICROMETER. 
51 
volving movement, fits rig- 23. 
the positive eye-piece 
d being the field-lens, 
and 8 the eye-lens. 
The slits h h, on oppo- 
site sides of G, (the ref- 
erences in Figs. 22 and 
23 are the same,) allow 
the micrometer with its 
mounting B B, to be in- 
troduced into G, and 
permit the graduated 
lines to be brought into 
the field of the eye-piece. 
C is an index, attached 
to G by the screw c, it may be taken off when the apparatus 
is not used as a goniometer. 
70. Method of using the Ooiiiometer. Bring the object 
into focus, near the centre of the field of the micrometer, 
applying the finger to the knob K, revolve the micrometer till 
the lines of its graduation are parallel to one side of the angle 
to be measured. Bevolve then, separately, the graduated 
circle till zero is brought to agree with the point of the index 
C. Then revolve again the micrometer by the knob K, until 
the graduation lines are parallel to the other side of the angle 
to be measured, when the index C will show the value of this 
angle. 
The micrometer lines are about iiich apart, but 
their value, when used for measurements with the different 
object-glasses and eye-pieces, must be ascertained by a stage 
micrometer and recorded in a table. 
71. Method of finding the value of lines in any Eye- 
piece Micrometer. For this purpose we must employ a stage 
micrometer, having lines ruled at some known distance, and 
this instrument should be of the very best quality, as the accu- 
racy of all our measurements wdth the eye-piece micrometer 
depend on the accuracy of the instrument with which their 
values are determined. 
CATALOGUE OF ACHROMATIC MICROSCOPES. 
0, OF [LL' UB« 
