98 
KIKTLEY F. MATHER 
the work of Wainwright/ Ransome^^ Wegemann,^ Stebinger^ 
and others. 
The writer is deeply indebted to his comrades of the United 
States Geological Survey, especially to K. C. Heald, E. Russell 
Lloyd and Eugene Stebinger, for much of the information 
assembled here. He is also obligated to the Bausch and Lomb 
Optical Company and to W. and L. E. Gurley for illustrations 
and tables reproduced from their catalogs and manuals. 
DESCRIPTION OF THE ALIDADE 
The telescopic alidade consists essentially of a telescope 
attached by a transverse axis to a base plate, one edge of which 
bears a fixed and approximately parallel relation to the line of 
sight, and so supported as to permit the telescope to be elevated 
or depressed in a vertical plane. Of the many different models 
on the market, that most suited to the needs of the petroleum 
geologist is the miniature” or explorer’s” alidade, designed 
in 1909 by H. S. Gale of the United States Geological Survey, 
some form of which is now produced by each of the leading 
makers of surveying instruments. This alidade is illustrated 
in figure 1. 
The telescope is a metal tube fitted with an object-glass at one 
end, an eyepiece at the other, and between the two a reticle 
holding cross-hairs. A diagrammatic longitudinal section of the 
telescope, showing the paths of light rays passing through it, 
forms figure 2. 
The object-glass ordinarily comprises a crown lens and a flint 
lens so shaped and arranged as to gather the rays of light from 
an object and form a small inverted image in the plane of the 
1 Wainwright, Plane-table manual. U. S. Coast and GeodeHc Survey, Kept, 
for 1905, App. 7, 1906. 
2 Ransome, F. L., The Plane-table in detailed geologic mapping. Econ. Geol. 
vol. 7, pp. 113-119, 1912. 
2 Wegemann, C. H., Plane-table methods as applied to geologic mapping. 
Econ. Geol., vol. 7, pp. 621-637, 1912. 
^ Stebinger, Eugene, Control for geologic mapping in the absence of a topo- 
graphic base map. Econ. Geol., vol. 8, pp. 266-271, 1913. 
