The nickel-plated ring will catch enough light fo accomplish 

 the desired result. It is neither necessary nor desirable that the 

 reflected light shall be concentrated at the diaphragm. Such a plan 

 would so flood the field that the dimly visible object would be 

 obscured. The reflector ring is made narrow partially on this 

 account and for the further desirable purpose of keeping the aperture 

 and light gathering capacity of the objective as large as possible. 

 A small opening defeats the purpose for which the larger apertures 

 are calculated. (See pp. 78 and 79). 



The reflector ring can be removed if desired, and by turning 

 the opening downward in an emergency the tube can be used as an 

 ordinary sunshade. 



Electric Head Light 



Artificial illumination of the diaphragm may also be accom- 

 plished by perforation of the horizontal axis, as described on p. II 1. 

 The source of light is represented there by a candle but the larger 

 theodolites are usually equipped with a bronze lamp mounted on a 

 bracket In more recent years the dry battery electric bulb lamp 

 has come into favor on account of its relatively greater luminosity 

 and cleanliness. 



Fig. no 



On page 181 we show in connection with the illustration of a 

 small mining theodolite, that of an electric hand magnifier which 

 could be used not only for reading the verniers in the dark, as 

 originally intended, but for axial illumination as described. 



For mining engineering or rescue work or for inspecting dark 

 forms in construction work, the Electric Head Light shown in Fig. 

 1 10 is especially well designed to secure a maximum illumination 

 for the current consumed. It may be used with any type of pocket 

 or belt primary or storage battery. If used with a 3-cell primary 

 or 2-cell storage battery, a 4-volt bulb is used; but if with a 3-cell 

 storage battery, a 6-\olt lamp is supplied. The burnished silver 

 parabola reflector is insulated against short circuits and will project 

 a beam of light over 100 yds. as well as a large circle of illumination 

 close to the wearer. Batteries burn twelve to twenty hours. 



IRS 



