A. ASHE ON THE DETERMINATION OF OPTICAL TUBE LENGTH. 153 



But the question obviously arises, where are these focal 

 planes situated, how are their positions to be located, and the 

 distance between them estimated ? 



The desire for information on these points will certainly not 

 be rewarded by any light the average microscopical text-book 

 may throw on the subject, for, whilst laying stress upon the 

 relationship existing between tube length and amplification, 

 they generally leave the reader very much to his own 

 resources as to the methods employed in solving the former 

 part of the problem. 



A recent article* in the " Journal of the Royal Microscopical 

 Society " on this subject is very interesting, but, unfortunately, 

 the method suggested, whilst perfectly accurate and thoroughly 

 scientific, incontrovertible in its theory and capable of giving 

 most excellent results in the hands of an expert, is yet, from 

 its very nature, far too complicated in the details of its manipu- 

 lation and abstruse in its mathematical principles to meet the 

 requirements of the average worker, whose possession of 

 apparatus is seldom of such an extent as to warrant his under- 

 taking an optical research of no small magnitude, and who 

 frequently hesitates to trust his conclusions to figures obtained 

 by the exercise of a long-forgotten skill in the solution of 

 algebraic equations. 



Under these circumstances I beg to call your attention to a 

 simple method of estimating the tube length which will not 

 involve the use of difiicult formulae or any apparatus beyond 

 an ordinary stage micrometer. 



It is based upon the increase in power obtained by extending 

 the draw-tube through some measured distance, and is carried 

 out thus :— 



A careful estimate is made of the power of the microscope 

 with the draw-tube pushed home as far as it will go, then 

 having determined this the eye-piece is withdrawn three or 

 four inches, the exact amount being noted and the increased 

 power of the instrument remeasured. 



We are now in possession of all the data necessary to calcu- 

 late — not the actual optical tube length, but its arithmetical 

 equivalent — a distinction to be observed, though the difference 

 is immaterial to the purpose in view. 



♦ ♦• R. M. S. Jouraal'" (1892), pp. 545, 546. 



