ALIDADES 



Fig. 28 



No. 9 Standard Topographic Alidade with Special Diagonal Eyepiece for 

 Direct and Indirect Sighting. 



The growing demand for alidades of quality and adaptability, 

 for a wide variety of purposes, is responsible for the various modified 

 types which we now manufacture. The standard model shown 

 above is the composite result of the most expert experience in the 

 field and shop. The full circle and the attached bubble tube of the 

 older types have given way to the edge reading arc and the re- 

 movable striding level. The telescope is mounted centrally in low 

 standards over the base plate for, though there be a slight error of 

 coincidence between the sight line and the fiducial edge, it is a 

 constant that will not affect the accuracy of the more careful work. 



It was previously thought that the telescope should be mounted 

 in an eccentric bearing so that it could be reversed in altitude in a 

 plane coincident with the edge of the ruler; but plunging the tele- 

 scope is unnecessary either in work or adjustment, and a sight line 

 that is removed \ l /t n (38 mm) from the ruling edge will not intro- 

 duce perceptible errors except in rare sights of a few yards distance. 



The Telescope is equal to that of the best theodolites. It is 

 of the inverting type, x23 or x32 as desired, provided w ith our new 

 interior focusing system, revolves longitudinally in the axis sheath 

 against 180 stops, and in elevation in a substantial standard whose 

 lower corners are rounded for convenience in handling. 



The Duplex Diagonal Eyepiece shown in the engraving is 

 a new design in which one may take observations either directly 

 through the principal ocular, when it is thrown into alignment with 

 the collimation axis, or indirectly through the cathetus extension, 



46 



