MANIPULATION OF THE TELESCOPIC ALIDADE 
113 
where s is the intercept on the rod between the outer hairs, and 
c the distance from center of instrument to objective, + c is 
practically a constant for a particular alidade, as it varies only 
Fig. 4. Stadia Rod in the Field of 
View at a Distance of 720 Feet 
Fig. 5. Stadia Rod in the Field of 
View at a Distance of 1740 Feet 
Fig. 6. Stadia Rod in the Field of View at a Distance of 3360 Feet 
with the focusing of the instrument. For ordinary purposes it 
may be taken as 1 foot, a space so short as to be altogether 
negligible in most work undertaken by the geologist. 
