55 SEC. 3. MEASUREMENT. 



in length, and divided throughout into millimeters, is inlaid into the prism. 

 Sliding along the same is a telescope, likewise fitted with a cylindrical -water- 

 level, the supporter of which is provided with a nonius indicating ^ mm. 

 Added to the same is 



Water-level, for regulating the direction of the prism. 



310. Cathetometer, so arranged as to be used as horizontal 

 measure ; can be unscrewed. Prof. Dr. Dove, Berlin. 



311. Cathetometer, by Breithaupt and Son, in Cassel, with 

 riding level. Polytechnic School in Cassel (Dr. E. Gerland). 



1 . The following improvements, contributing partly to more minute indica- 

 tions of the apparatus, partly allowing the correction of the several parts the 

 one to the other, have been added to the well-known construction of catheto- 

 meters. 



The firmly placed central 'axis, around which the long case with prism 

 turns, can be placed vertically by a special cylindrical water-level, indicating 

 10 minutes ; the latter is therefore fastened to that case independently of 

 other parts, in order that the vertical position of the axis required at very 

 fine measurements can be easily observed ; the more so, as all other exami - 

 nations are based on the correct adjustment of this water-level. The vertical 

 position of the axis is effected in the same manner as with an ordinary 

 levelling instrument, and any deviations of the water-level are corrected half 

 on the correction screw of the same, and half by the regulating screws of the 

 tripod. 



The prism, the inlaid silver scale of which, of 1 meter in length, is through- 

 out divided into millimeters, and fitted with a t vernier for -$ mm., can be 

 placed in a perpendicular and a horizontal position towards the axis of revo- 

 lution. For this purpose a specially constructed attaching or adjusting 

 water-level, which is fastened to a right angle, and can be turned, is placed 

 on the face, and the edges of the prism, and thereby the latter, which has at 

 the sides and behind regulating screws for that purpose, is corrected as re- 

 quired ; at this correction it is necessary to pay particular attention, by means 

 of water-level attached to it, to the exact perpendicular position of the axis 

 of revolution. 



After the telescope water-level has been previously examined by reversion 

 and adjusted, the rectangular position of the telescope towards the prism is 

 effected by screws, which allow a slight raising or lowering of the telescope 

 supporter. If this is done, the bubble of the telescope water-level will 

 remain unchangeably in the centre at the turning of the whole instrument 

 around its central axis, as well as at the upward and downward motion of the 

 shifter. 



A very severe proof consists in sighting with the telescope a distant object, 

 the telescope being reversed in its sockets, and the apparatus turned round 

 180, at which manipulation the former object must be intersected again by 

 the ocular cross what must always be the case with an exactly constructed 

 and well rectified instrument. 



The unalterable cross in the ocular is cut in glass, in order to prevent 

 hygroscopical and other interruptions. F.or the purpose of obtaining the 

 rectangular position of the telescope, the supporter may also be placed with 

 one end between points, while an elevation screw is fixed to the other. The 

 essential point for effecting the before-mentioned correction by employing the 

 attaching or adjusting water-level consists simply in adjusting the water-level 

 axis exactly to the leaning face by means of the correction arrangement 

 marked a in the drawing. The proof is effected by reversing the angle 



