316 MAURICE A. AINSLIE OX THE MEASUREMENT OF 



is proportional to this " optical tube-length," and since different 

 eye-pieces, when in position in the tube, very often have their 

 lower focal planes at widely different distances from the top 

 thereof, it follows that the magnification due to the objective is 

 often altered by change of eye-piece. This is, in fact, always the 

 case if any refocusing is required on change of eye-piece ; and 

 for this reason, as well as for the purpose of retaining the tube- 

 length (on the correctness of which the proper performance of the 

 objective so much depends) it is always desirable, on changing 

 the eye-piece, to restore the focus, if necessary, by alteration of 

 the tube-length. 



But quite apart from this, there is another reason why the 

 powers given by different combinations do not always follow the 

 strict rule of proportionality as stated by Mr. Bale ; the magnify- 

 ing power of the objective, as stated above, is strictly proportional 

 to the " optical " tube-length, as mentioned above, and not to 

 the " mechanical " tube-length, which is the actual length of the 

 tube, measured from top to bottom. The optical tube-length of 

 course varies with the position of the upper focal plane of the 

 objective ; in one objective this may be as much as 50 mm. below 

 the bottom of the tube, while in another it may be as much as 

 50 mm. above it ; so that, even with the actual length of the tube 

 unaltered, tliere may be a variation, with different objectives, of 

 as much as 100 mm. in the optical tube-length, quite apart from 

 the variation introduced by change of eye-piece. It is obvious 

 that if the optical tube-length is thus altered by change of ob- 

 jective, the power of the objective ceases to be inversely pro- 

 portional to its equivalent focal length ; or, to put it in more 

 popular language, a "quarter" will not necessarily magnify 

 four times as much as an " inch." 



It thus follows that it is not such a simple matter as it might 

 seem to compute the magnification of one combination from that 

 of another ; indeed, it is impossible to do so with any real approach 

 to accuracy unless the positions of the upper focal plane of the 

 objective, and of the lower focal plane of the eye-piece, are taken 

 into account. 



An actual example may help to make this clearer. I have 

 measured the magnifying powers with two objectives — the Leitz 

 No. 1, focal length 39*4 mm., and tlie Zeiss 12-mm. Apo., focal 

 length 12'6 mm.: and two eye-pieces, the Holos "10," actual 



