MAGNIFYING POWER : A NOTE ON MR. BALE's PAPER 317 



power 9*4, and the Zeiss Compensatiug " 8," actual power ITS. 

 The results are given in the following table : 



The upper focal plane of the Leitz No. 1 objective is 28'5 mm. 

 above the lower end of the tube ; that of the Zeiss 12-mm. is 

 19"8 mm. heloiv the same. It follows that the optical tube-lengths 

 were as follows in the three cases : 



Leitz No. 1 . . . . 141-5 123-5 UTo 



Zeiss 12-mm 189*8 171*8 189-8 



and the magnifying powers of the objectives by themselves 

 (optical tube-length divided by focal length of objective) were : 



Leitz No. 1 . . . . 359 3-14 3*59 



Zeiss 12-mm lo'OS 13-63 15'05 



(The difference between the second and third columns is due to 

 the fact that for the second column the refocusinoj necessitated 

 by change of eye-piece was performed in the ordinary way by the 

 coarse adjustment ; for the third column, by pulling out the 

 draw-tube, so as to adjust the lower focal plane of the eye-piece 

 to the image, the position of the latter being unaltered ; in other 

 words, the " optical " tube-length is the same for columns one 

 and three, but not for columns one and two, as will indeed be 

 seen from the table.) 



It will be obvious from the above table : 



(1) That the magnifying power with the same eye-piece is not 

 inversely proportional to the focal length of the objective ; this 

 is due to the great difierence (in this case 19*8 + 28'5, or 48'3 

 mm.) between the positions of the upper focal planes of the 

 two objectives considered. In the present case the ratio of the 



