The Focal Length Apparatus. 



Used to determine the focal length of an objective from the optical center, as well as from the 

 erand principal point. The latter by attaching a micrometer microscope at one end of the grad 

 n**d bar and measuring with it through the objective the wire interval of the coilimator A. 



< . I.. 15<-rj;<'r & Sons' Auxiliary Apparatus, 

 lifted daring the contraction of their Instruments of Precision. 



