The Amplex Model shown with a hinge is especially de- 

 signed so that the telescope may transit at the eye-end while this 

 device is applied, in order to correct for possible errors in the 

 horizontal axis. In connection with our Tachymeters such a 

 mechanical provision is not necessary, for the telescopes transit at 

 the objective end. The focusing is accomplished with the knurled 

 head at the base. 



Fig. 114 



The Duplex Model is so designed that the tube slides up 

 and down at will between stops on the swivel adapter ring so that 

 the prism may be instantly thrown into, or removed from the sight 

 line as required. This arrangement permits one to transfer sights 

 between the zenith and the horizon with minimum effort and 

 chance of error. This also makes it an ideal construction for the 

 Standard Adjustment as described on p. 72. The large milled 

 swivel head is used to attach the appliance to the telescope without 

 turning the whole device. Some engineers prefer to connect the mine 

 traverse with the surface survey by carrying the reference line upward 

 on account of the advantage of sighting against daylight. If the shaft 

 is dry and upcast, this system possesses sundry advantages. 



The Objective Prism of the ordinary variety, as shown in 

 Fig. 115, is now nearly obsolete. It was used by Steinheil for 

 astronomical observations in 1847, 

 and Prof. Stampfer applied it to a 

 wye level in 1852 for nadir sight- 

 ing in mines. The casual ap- 

 plication of such a prism does not 

 guarantee a constant deflection angle, 

 and any error in the relative position 

 us of the reflecting surface to the in- 



cident beam will be doubled in the reflected ray. To illustrate: let 

 ABO, Fig. 116, be the correct position of the prism to project the 

 incident ray, MR, at exact ri^ht angles to its original direction; 



192 



