242 The Microscopic Compass. 
and to the magnetic axis of the needle. The pieces AA are bent — 
as in Fig. 2. so as to raise the rim even with the point of suspen- _ 
sion in the cap. C is the brass cap riveted through the needle, 
and D an agate cap fastened into its socket, by burnishing the 
brass over its edge in the lathe. The rim is on fiftieth of an inch 
thick, and divided very accurately into degrees. The figures num- 
bering the degrees, must be reversed in the manner of types, 
because they are to be read in the reflector which will rectify them. 
Fig. 2. This is in general a vertical section of the whole instrument _ 
in the plane of the sights. VVV is the box turned out of cast brass — 
or soldered out of thick sheet brass. G is the glass resting ona pro- 
jection below and held in by aring above. The sight KS has a joint 
at K by which it can be turned down on the glass G. M is the re- 
flector previously described. I is a screw passing through the brass 
back which protects the silvered part, and screws against a thin plate 
interposed between it and the glass, to keep the latter firmly in its 
place. Lis the lens set in brass and fastened to the sight by two 
screws below, each near the outer edge of the sight. The brass is 
slitted on the screws soas to allow of an adjustment of the micro- 
scope. The lens should be seven tenths of an inch in diameter, and if 
necessary should be ground off on the side next the sight so as to bring 
the axis over the rim R, or the axis may be a little inclined so- that 
the ray RM shall not be perpendicular, but shall strike the reflector, — 
say, at the screen I. H isa steel pivot screwed into the bottom of 
the box with a ‘flange’ at X squared for unscrewing. PHP isa 
Spring to raise the needle against the glass from the point. W is the 
serew by which it is raised. The dotted line TTTT represents 
a brass cover which closes the whole instrument, the sight KS and 
the reflector M being shut down by means of their joints KJ. The 
. Compass thus closed has the appearance of a pocket sextant. ~ 
the top and bottom of this box is the milled projection, which is of 
use in opening and holding it. Fig. 4. has already been described. 
An making the instrument the artisan should be careful soto construct = 
Mt, that the plane of the sights may pass through the point of the pi- ee 
vot and that the hinge of the reflector may have its axis exactly per- 
Pendicular to that plane. The ends of the needle should not eome 
within the eighth of an inch of the box, as it may otherwise be influen-_ = 
pees the magnetism of the brass. I have not found a specimen 
_ hammered brass which was not magnetic when tried by a delic 
test needle. Its magnetism is however destroyed by heating it 
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