W. LeConte Stevens — Microscope Magnification. 57 



eye-lens by § does not therefore give the equivalent focal 

 length of the combination. The approximation, as shown in 

 colurns 5 and 6 is moderately good in eye-pieces A 2 and C, but 

 by no means so in B t and B 2 . In column 5 the value of F was 

 computed by formula (3), and the results translated into inches 

 for column 7. Column 8 shows the percentage of error in the 

 nominal equivalent focal lengths of the eye-pieces, and column 

 9 shows their actual magnifying power. Each of the data of 

 columns 2 and 3 is the mean of five independent measure- 

 ments ; but the results in column 9 are affected with a 

 probable error greater than what should be expected if F could 

 have been obtained directly from f alone. 



The maker of an eye-piece ought certainly to know how to 

 test his work after it is finished. He has the right to use any 

 formula in construction that experience has shown to be valu- 

 able. But in every case the value of F ought to be determined 

 accurately by him, and labeled on the mounting of the eye- 

 piece, not in whole inches or aliquot parts of an inch, but in 

 decimal parts of an inch, or, still better, in millimeters. The 

 scientific world is familiar enough with the metric system to 

 warrant the abolition of other systems, at least in the construc- 

 tion of all new instruments. 



Table II. 



1. 



2, 



3. 



4. 



5. 



6. 





I 



ra 



/. mm. 



/. inches. 



e% 



W. 3 



315 



3-93 



51-0 



2-01 



+ 50 



B. 2 



303 



6'25 



36-1 



142 



+ 41 



R. H 



300 



7-10 



32-4 



1-28 



+ 17 



B 1 



293 



9-33 



25-6 



101 



-1 



B.f 



284 



13-50 



18-2 



■716 



+ 5 



W. £ 



296 



14-00 



184 



•724 



+ 4 



B.| 



288 



18-80 



13-5 



•531 



-6 



o-i 



290 



55-00 



5-1 



•202 



+ 24 



B. i 



283 



58-00 



4-7 



•185 



+ 7 



W. -h 



277 



1150 



2-36 



•093 



-10 



B. -h 



287 



170-0 



1-68 



•066 



— 5 



In applying formula (12) to the determination of the focal 

 lengths of objectives it is found that the labeling of these is in 

 many cases very erroneous. In the paper to which reference 

 has already been made Professor Cross gave his measurement 

 of more than thirty objectives from various sources. In one 

 case, an objective, marked -^ inch, should have been marked J 

 inch. The measurements made by the present writer and re- 

 corded in Table II above, may give some idea of current errors 



