SURVEYING INSTRUMENTS. l 



The telescope is carried by a ring, L, attached to a stem, 

 e, called tlie up-and-down piece, which can be raised or 

 lowered by turning the milled screw, M : its use is to place 

 the telescope so that the field of view may be bisected by 

 the line on the horizon-glass that separates the silvered from 

 the unsilvered part. This is important, as it renders the 

 object seen by reflection, and that by direct vision, equally 

 bright ; two telescopes and a plain tube, all adapted to the 

 ring, L, are packed with the sextant, one showing the objects 

 erect, and the other inverting them ; the last has a greater 

 magnifying power, showing the contact of the images much 

 better. The adjustment for distinct vision is obtained by 

 sliding the tube at the eye-end of the telescope in the inside 

 of the other ; this also is the means of adapting the focus 

 to suit different eyes. In the inverting telescope are placed 

 two wires, parallel to each other, and in the middle of the 

 space between them the observations are to be made, the 

 wires being first brought parallel to the plane of the sextant, 

 which may be judged of with sufficient exactness by the 

 eye. When observing with this telescope, it must be borne 

 in mind, that the instrument must be moved in a contrary 

 direction to that which the object appears to take, in order 

 to keep it in the field of view. 



Four dark glasses, of different depths of shade and color, 

 are placed at K, between the index and horizon glasses ; 

 also three more at N, any one or more of which can be 

 turned down to moderate the intensity of the light, before 

 reaching the eye, when a very luminous object (as the sun) 

 is observed. The same purpose is effected by fixing a dark 

 glass to the eye-end of the telescope: one or more dark 

 glasses for this purpose generally accompany the instrument. 

 They, however, are chiefly used when the Sun's altitude is 

 observed with an artificial horizon, or for ascertaining the 

 index error, as employing the shades attached to the instru- 

 ment for such purposes, would involve in the result any 

 error which they might possess. The handle, which is shown 

 at 0, is fixed at the back of the instrument. The hole in 

 the middle is for fixing it to a stand, which is useful when 

 an observer is desirous of great steadiness. 



Of the adjustments. The requisite adjustments are the 

 following : the index and horizon-glasses must be perpen- 

 dicular to the plane of the instrument, and their planes 

 parallel to each other when the index division of the vernier 



