ASTRONOMICAL INSTRUMENTS. 1?3 



rated, admits the rays of light to the centre of the tele- 

 scope-tube, where, falling on a diagonal reflector, they are 

 reflected to the eye, and illuminate the field of view ; the 

 whole of this contrivance is precisely similar to that de- 

 scribed as belonging to the transit instrument. 



The vertical circle is usually divided into four quadrants, 

 each numbered, 1, 2, 3, &c., up to 90, and following 

 one another in the same order of succession ; consequently, 

 in one position of the instrument, altitudes are read off, and 

 with the face of the instrument reversed, zenith distances ; 

 and an observation is not to be considered complete till the 

 object has been observed in both positions. The sum of the 

 two readings will always be 90, if there be no error in the 

 adjustments, in the circle itself, or in the observations. 



It is necessary that the microscopes, O, and the centre 

 of the circle, should occupy the line of its horizontal diame- 

 ter ; to effect which, the up-and-down motion (before spoken 

 of) by means of the screws, b b, is given to the Y's, to raise 

 or lower them, until this adjustment is accomplished. A 

 spirit-level, P, is suspended from the arms which carry the 

 microscopes : this shows when the vertical axis is set per- 

 pendicular to the horizon. A scale, usually showing seconds, 

 is placed along the glass-tube of the level, which exhibits 

 the amount, if any, of the inclination of the vertical axis. 

 This should be noticed repeatedly whilst making a series of 

 observations, to ascertain if any change has taken place in 

 the position of the instrument after its adjustments have 

 been completed. One of the points of suspension of the 

 level is movable up or down, by means of the screw, /, for 

 the purpose of adjusting the bubble. A striding level simi- 

 lar to the one employed for the transit instrument, and used 

 for a like purpose, rests upon the pivots of the axis. It 

 must be carefully passed between the radial bars of the 

 vertical circle to set it up in its place, and must be removed 

 as soon as the operation of levelling the horizontal axis is 

 performed. The whole instrument stands upon three foot- 

 screws, placed at the extremities of the three branches which 

 form the tripod, and brass cups are placed under the spheri- 

 cal ends of the foot-screws. A stone pedestal, set perfectly 

 steady, is the best support for this as well as the portable 

 transit instrument. 



Use of the Altitude and Azimuth Instrument. This is the 

 most generally useful of all instruments for measuring an- 

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