54 W. LeConte Stevens — Microscope Magnification. 



.-.PQ = |p'Q'. 



The focal plane of the objective is hence midway between 

 the eye lens and its focal plane ; and the diameter of the 

 image actually viewed with this lens is two-thirds of that 

 which would have been formed if the field lens had been 



absent. If we assume -~ as the magnifying power of the 



objective when no field lens is used, the interposition of this 



2 10 

 lens reduces it to - , — m This reduction of magnification is 



more than offset by the well known advantages which the 

 field lens confers. Introducing the proper correction in 

 formula (1), this becomes 



„ 2 100 



M =¥^ V- 



Formula (2) implies a knowledge of the focal length of the 

 objective and of only the eye lens. To find the equivalent 

 focal length of the eye-piece combination, let F stand for this 

 length, f and f" for those of eye lens and field lens respect- 

 ively, and d for the interval between these lenses. Then the 

 usual formula for the combination is 



1 — 1 l cl 



In this case y" =3/'' and d=2f. Substituting, we have 



/'=f F - .... (*). 



Introducing this value of f in formula (2), the result is 



-.r 100 \ . 



m =f7 ( 5 )- 



The value of f is labeled on the mounting of the objective, 

 and that of F is easily obtained by applying formula {4:),f r 

 being found without calculation, as suggested above, if great 

 accuracy is not required. 



In formula (5), 100 is the product of two factors. One of 

 them is the assumed distance at which distinct vision with the 

 unaided eye is most easily attained! It may be taken as 250 

 millimeters, which is very nearly 10 inches. The other is the 

 distance from the focal plane of the objective to what we may 

 provisionally call its optical center. If we make this last dis- 

 tance our definition of tube length, use for it the symbol T, 

 and let D stand for the distance of distinct vision, our formula 

 becomes, 



