“axis of the instrument till by viewing the — 
. the crystal — the distance of its focus 
108 Measurement of angies under ihe Compound Microscope. 
faces coincident with the axis of the compound body: it can 
readily be determined by the inch lens which is to be supported 
on a stand in the position seen in fig. 1, when this is nearly ac- _ 
complished. ‘Two hairs crossing each other at right angles are — 
to be fastened to the end of the brass tube in which the lens is | 
set. If now one of the faces under consideration be made to re- 
flect the light so as to be seen brilliantly illuminated by the inch 
lens, the condenser removed and a second lens of about two inches 
focal length, (the field lens of one of the eye-pieces will answer 
for this purpose,) be held by the hand behind the inch lens, after 
a little trial a position can be found, where, instead of the illumin- 
ated face of the crystal, a distinct image ‘of the flame, more or 
less inclined to the perpendicular cross- hair, will be seen. The 
wax is then to be bent till the image of the flame assumes an 
upright position and coincides with the perpendicular cross-hair, 
and the same is to be done with the other face. It is not neces- 
sary that the two-inch lens should be supported otherwise than 
by the hand, the cross-hairs not being attached to it; their posi- — 
tion in relation to the i image of the flame does not change with : 
the motion of the hand. This is very convenient, enabling the ‘ 
observer to view at pleasure and without loss of time the face of 
the eryatal or the reflected image of the flame. A little rod of 
wood ,'; of an inch in diameter is to be su pported ina horizontal 
position “between the flame and the crystal: it answers the pur- 
per of the “ window bar” in the ordinary form of the ca i 
wT he erystal can thus be turned on the ot. 
image of the flame through the two-inch 
lens, the bar and horizontal cross-hair are 
seen to coincide, &c., the farther manipula- 
tion being the same as in the common mode 
of using the reflective goniometer. This 
method in practice will be found easy, and it 
is evident that any desirable amount of ac- 
curacy can be attained. Sometimes it may 
be found bear t not to.remove the condenser ; 7S : 
it then should be bro somewhat nearer Ay 
ee 
when the image of the flame will b e seen as 
before : in this case the wax should be pre- 
— meckeriod, 
Shey 
made to coincide as nearly as possi 
4 * 
neh oe 
a + 
Re elit € 
