The meant employed to concentrate an image produced by a 

 fixed lens upon an immovable diaphragm may be understood by an 

 inspection of Fig. 16, which is reproduced from a discussion on the 

 tele-photo lens by Baker.* The principle is not new but Mr. H. 

 Wild of Jena, was the first to use it for this purpose in 1909.** 



Naturally the image of a distant object would be produced at 

 the principal, or shortest, focus of the objective, but interposing the 

 negative lens at the proper position throws the image back the 



J 

 I 



*>-- Equivalent Jbcal distance - 



Pig if 



required distance, producing the same effect as though an objective 

 of equivalent focal length had been racked out to the position shown 

 by the lens in dotted outline at A. 



In effect, we have two telescopes combined in one, and 

 avowedly two optical axes which must be collimated to coinci- 

 dence so that sights can be taken in either direction along the same 

 straight line. This adjustment is the only one upon which the 

 accuracy of the instrument depends, so that it is permanently fixed 

 by us. That the operator may reassure himself, however, in regard 

 to the collimation adjustment, a set of improvised wyes are provid- 

 ed; but they are not intended for use in the bubble adjustment. 



There being no collimation adjustment to concern the operator, 

 proceed with the Bubble Test as follows: 

 ^et up instrument firmly with one set of leveling screws in the 



general direction of the rod. 



Bring bubble carefully to the center of its run with the Vertical 

 Control by turning the knurled head at either end of the base 

 bars, and take a reading on a rod about 100, or 150, ft. from 

 the instrument. 



Tip the mirror in its pinions, remove ocular, leave vertical axis 

 clamped but open key in the clamp-and-tangent movement; re 

 volve telescope end-for-end and re-engage the key in the seat- 

 ing provided. Replace ocular and re-focus on rod. 

 If the bubble has run off Center bring it back with the Vertical 



Control and take a second reading. 

 If the readings are identical, the bubble is in adjustment. 



' Thick Isns Oplu>, A. I.. Itakft, 19 12, p. 90. 

 >rl ju, ln>i>H>n. 



31 



