must be removed, and the diaphragm as well, with an improvised 

 spring wire handle. This will require skill, care and patience but 

 it can be accomplished if the occasion demands. It would be sim- 

 pler to remove the objective, rack out the focusing lens barrel and 

 also remove that, then improvise a swab and very carefully clean 

 the inner surface by this means. (See also top of p. 102). 



Platinum Wires have been used considerably in the past in 

 order to overcome the hygrometric properties of spider webs. The 

 platinum wire is soldered into a silver tube and drawn down to the 

 uttermost fineness. When the silver coating is eaten off with nitric 

 acid, the platinum core must not exceed .005 mm in diam. For a 

 time they were very popular, but it is not so easy to fix so fine a 

 metallic filiament to another metallic surface and the platinum 

 seems to loose some of its ductility and breaks off under shocks. 

 We frankly confess that w r e have never had much success with the 

 scheme. For the Nos. 90 and 92 Alidades, for instance, the glass 

 diaphragms are more permanent and even in humid climates are 

 dependable and altogether satisfactory. 



Fig. 82 



Diaphragm Illumination for mining and night work is 

 accomplished by the Illuminator Tube described on p. 187, and by 

 axial perforation with either oil-lamp or electric illuminant. The 

 oil-lamp system is illustrated in connection with the cut of the 

 7-inch Transit Theodolite on page 143 and the electric dry bat- 

 tery system in connection with the 8-inch Meridian Theodolite 

 illustrated on p. 149. They differ only in the matter of light source; 

 the principle is identical and may be understood by inspection of 

 Fig. 82. 



144 



