C being at the center of the instrument, the angle ACB is 

 made equal to the proportion of 1 :100, or 34' 23", when CA or CB~ 

 100 and AB 1. The objective, however, is calculated so that the 

 focus of such rays fall well within the distance CO at F, beyond 

 which they diverge, meeting the anallatic lens at ai and bi. 



Fie. 68 



The anallatic lens, L, is an immovable achromatic combination 

 of a focal length equal to LF FO and being also a convergent lens, 

 the rays approaching from F are thrown towards the ocular, E P, 

 in parallel lines so as to pass through the stadia wires at the dia- 

 phragm, D. This will be so whatever the distance P L, due to 

 racking the ocular in the process of focusing. 



It will be apparent that this combination of purpose consider- 

 ably shortens the focal length of the objective so that the magnifi- 

 cation is reduced somewhat, as the anallatic lens is moved toward 

 O. This principle offers a favorable means of adjusting a certain 

 wire interval to suit the conditions of the field instead of attempt- 

 ing to make the stadia wires adjustable according to the generally 

 condemned expedient discussed on p. 107. It stands to reason, 

 however, that the anallatic lens should be fixed at its principal 

 focus from the point F, or the extreme rays, passing out toward the 

 ocular, will not be strictly parallel and the readings will be incon- 

 sistent. 



For the reason that the focal length of the objective is consider- 

 ably reduced, the same flatness of field or magnification is not to be 

 expected. Generally, an ocular of very short focus has been em- 

 ployed for this system to restore the power to about X20 or else 

 the telescope has been increased in length so that the distance FO 

 may be equal to that of a normal objective. The point F being 

 fixed for infinity, the distance FC will gradually decrease as the 

 rod approaches the instrument until a point will be reached when 

 F and C are coincident. The minimum range may be determined 

 by experiment, but it is to be understood that the anallatic method, 

 as applied to instruments of average size, is generally unreliable 

 under a 50-ft. or 16-m. range. 



149 



