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



reflector having a large hole in its centre, does not interfere 

 with the rays passing clown the telescope from the object, 

 and thus the observer sees distinctly both the wires and the 

 object at the same time ; when, however, the object is very 

 faint (as a small star), the light from the lamp would over- 

 power its feeble rays. To remedy this inconvenience, the 

 lamp is so constructed, that by turning a screw at its back, 

 or inclining the opening of the lantern, more or less light 

 may be admitted to the telescope, to suit the circumstances 

 of the case. 



The telescope is furnished with a diagonal eye-piece, by 

 which stars near the zenith may be observed without incon- 

 venience. 



Of the adjustments. Upon setting the instrument up, it 

 should be so placed that the telescope, when turned down 

 to the horizon, should point north and south as near as can 

 possibly be ascertained. This of course can be but approx- 

 imate, as the correct determination of the meridian can only 

 be obtained by observation, after the other adjustments are 

 completed. 



The first adjustment is that of the line of collimation. 

 Direct the telescope to some small distant well-defined ob- 

 ject (the more distant the better), and bisect it with the 

 middle of the central vertical wire ; then lift the telescope 

 very carefully out of its angular bearings, or Y's, and re- 

 place it with the axis reversed ; point the telescope again to 

 the same object, and if it be still bisected, the collimation 

 adjustment is correct ; if not, move the wires one half the 

 error, by turning the small screws which hold the diaphragm 

 near the eye-end of the telescope, and the adjustment will 

 be accomplished ; but as half the deviation may not be cor- 

 rectly estimated in moving the wires, it becomes necessary 

 to verify the adjustment by moving the telescope the other 

 half, which is done by turning the screw a ; this gives the 

 small azimuthal motion to the Y before spoken of, and con- 

 sequently to the pivot of the axis which it carries. Having 

 thus again bisected the object, reverse the axis as before, 

 and if half the error was correctly estimated, the object 

 will be bisected upon the telescope being directed to it ; if 

 not quite correct, the operation of reversing and correcting 

 half the error, in the same manner, must be gone through 

 again, until, by successive approximations, the object is found 

 to be bisected in both positions of the axis ; the adjustment 



