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 whale instrument 

 in the plane of the sights. WV is the box turned out of cast brass 

 or soldered out of thick sheet brass. G is the glass resting on a pro- 

 jection below and held in by a ring 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. L is 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 so as 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 T. H is a steel pivot screwed into the bottom of 

 the box with a ' flange' at X squared for unscrewing. PHP is a 

 spring to raise the needle against the glass from the point. W is the 

 screw 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. At 

 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. 

 In making the instrument the artisan should be careful so to construct 

 it, that the plane of the sights may pass through the point of the pi- 

 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 come 

 v/ithin the eighth of an inch of the box, as it may otherwise be influen- 

 ced by the magnetism of the brass. I have not found a specimen 

 of hammered brass which was not magnetic when tried by a delicate 

 test needle. Its magnetism is however destroyed by heating it to a 



