Rarnsden ocular which gives a positive image of the field. The 

 lines and figures appear in their proper relation elect ;.s it \\cu-. 

 The reason this combination of lenses slums objects inverted in a 

 telescope is because the image of the field is projected beyond the 

 plane of the diaphragm in an inverted position by the objetthe. 

 The drawing shows the magnifier mounted nearly normal to the 

 bevel of the limb, but some engineers prefer to have the magnitieis 

 slant a little (as we are required to build them for a vertical circle, 

 or a flat limb) in order to sight obliquely along the lines in select- 

 ing the position of coincidence. The objection to the plan is that 

 the upper and lower portions of the field are not in perfect focus at the 

 same time, but prismatic cover glasses would, in a great measure, 

 remedy this. 



Our magnifiers are mounted on radial arms that move in short 

 arcs following the curvature of the reading edge as shown in Figs. 

 73, 77 and 81. The method of mounting magnifiers for the vertical 

 limb is also shown in these illustrations. 



The Steinheil Heliotrooe 



Fig. 76 



In surveys limited in area, e\en to the confines of municipal 

 boundaries, the atmo.spheie is frequently so dense that ordinary 

 targets are either indistinct or invisible, in all such cases the 

 heliotrope is a serviceable accessory for picking up a sunln-iun and 

 projecting it into the theodolite precisely as though the target were 

 a luminous object. 



Otherwise the heliotrope is not much used for distances under 

 10 miles for the heliotroper must be expert and attentive, and unless 



135 



