146 



VI. — Eye-pieces for the Microscope. 

 By Edward M. Nelson. 



{Bead February 19, 1908.) 



Having been informed by Messrs. Zeiss that the glass 0*82 had 

 been taken out of Messrs. Schott and Co.'s list, I selected another 

 glass, viz. that used for the prisms in the best quality of binoculars, 

 and have recomputed the table of eye-pieces for that glass. This 

 glass is of a permanent nature, clear, and of low dispersion, so it is 

 in every way suitable for eye-pieces. 



To repeat the explanation of terms, s is the radius of the surface 

 of the eye-lens next the eye, and r the radius of that towards the 

 object glass, b being the diameter of the eye-lens ; S, R, and B 

 have a similar meaning with reference to the field-lens ; d' is the 

 distance between the surfaces of the lenses, h the diameter of the 

 hole in the diaphragm, t is the distance the incident surface R is to 

 be below the top of the tube of the Microscope, and F is the 

 equivalent focus of the eye-piece. 



For the formulas upon which these eye-pieces have been calcu- 

 lated, the reader is referred to the original paper in this Journal, 

 1900, p. 165. The following are additional formulas to those given 

 in that paper : — 



b = 0-575/'; B=-i* ; q= f *J . ; 



i _ / / -r-/ - d 



10 



t = q-(l- I)F-0-3in. 



These formulae give the theoretical values of b and B ; in 

 practice either b must be a little reduced, or B increased. In 

 Table I., for the short tube, alternative values of b, h, B, and d' are 

 given for R.M.S. standard gauge No. 1, and in Table II. values are 

 given for R.M.S. gauge No. 4. 



Instead of designating the eye-pieces by letters, or by numbers, 

 such as I., II., III., etc., other numbers are placed at the head of 

 the columns. These numbers represent the magnifying power of 

 the eye-piece when a certain tube-length is employed. As every 

 object requires a different tube-length, the magnifying power of 

 the entire Microscope is a variable quantity ; consequently, when 

 accuracy is required, the magnifying power must be determined for 

 each separate case, but for rough estimations the number at the 

 head of each column will be useful as a multiplier. 



