ASTRONOMICAL INSTRUMENTS. 127 



cf the vertical standards, D D. One of the Y's possesses a 

 small motion in azimuth, communicated by turning the 

 scre\v, a ; in these Y's the telescope turns upon its pivots. 

 But, that it may move in a vertical circle, the pivots must 

 be precisely on a level with each other, otherwise the tele- 

 scope will revolve in a plane oblique (instead of perpendicu- 

 lar) to the horizon. The levelling of the axis, as it is called, 

 is therefore one of the most important adjustments of the 

 instrument, and is effected by the aid of a spirit-level, E, 

 which is made for this purpose to stride across the telescope, 

 and rest on the two pivots. 



The standards, D D, are fixed by screws upon a brass 

 circle, F, which rests on three screws, bed, forming the 

 feet of the instrument, by the motion of which the opera- 

 tion of levelling is performed. The two oblique braces, 

 G G, are for the purpose of steadying the supports, it being 

 essential for the telescope to have not only a free but a 

 steady motion. On the extremity of one of the pivots, 

 which extends beyond its Y, is fixed a circle, H, which 

 turns with the axis while the double vernier, e e, remains 

 stationary in a horizontal position, and shows the altitude 

 to which the telescope is elevated. The verniers are set 

 horizontal by means of a spirit-level, /, which is attached 

 to them, and they are fixed in their position by an arm of 

 brass, g, clamped to the supports by a screw at h. The 

 whole of this apparatus is movable with the telescope, and 

 when the axis is reversed, can be attached in the same man- 

 ner to the opposite standard. 



Near the eye -end, and in the principal focus of the tele- 

 scope, is placed the diaphragm, or wire-plate, which, in the 

 theodolite or levelling telescope, need only carry two cross 

 wires, but in this instrument it has five vertical and two 

 horizontal wires. The centre vertical wire ought to be fixed 

 in the optical axis of the telescope, and perpendicular with 

 respect to the pivots of the axis. It will be evident, upon 

 consideration, that these wires are rendered visible in the 

 daytime by the rays of light passing down the telescope to 

 the eye ; but at night, when a very luminous object, as the 

 moon, is observed, they cannot be seen. Their illumination 

 is therefore effected by piercing one of the pivots, and ad- 

 mitting the light of a lamp fixed on the top of one of the 

 standards, as shown at I ; which light is directed to the 

 wires by a reflector placed diagonally in the sphere B ; the 



