The Short Focus Lens 



The Short Focus Lens is an additional lens applied to the 

 objective so that the focal length of the combination can be s; 

 iently reduced to observe objects in cramped places that are very 

 near to the instrument. For this purpose it is presupposed that the 

 collimation adjustment in the theodolite is perfect for all distances 

 particularly short ones. (See top of pp. 7 and 67). 



Fig. IIT 



It may be rationally assumed that unless some special precau- 

 tions are observed, the casual application of a second objective 

 lens will create a new optical axis and modify the collimation 

 adjustment. For observing horizontal angles, however, an error in 

 the vertical plane only, as in cut III Fig. 112, would be negligible; 

 but some rapid and accurate method of adjustment must be devised 

 as explained below. 



The4'< and 5-inch erecting telescopes focus normally down 

 to within considerably less than 5 ft. from the objective, as indicated 

 in the reference table. These, or any other telescope, could be made 

 to focus normally upon an object much closer by extending the 

 focusing rack movement; but for mining work particularly there is a 

 limit to such an expedient, fixed by the necessity of transiting the 

 telescope through the standards at the objective end. If one wished 

 to reduce the outer conjugate focal length of a 4Ji-in. inverting 



189 



