W. LeConte Stevens 

 with the camera lucida 



Microscope Magnification. 59 



The next two columns result from 



100 

 applying the formula M=^p and reducing to millimeters; 



in column 4 the values of F and f have been taken from 

 Tables I and II, and in column 5 they are the nominal focal 

 lengths, as indicated by the manufacturers. 



Table III. 



1. 



2. 



3. 



4. 



5. 



Combination. 



il = mm'. 



■ Camera. 



! S£=™ 



M = I°A 















F/ 



F/ 



B, 



X 



W 



3 



30-8 



30-4 



34-14 



2222 



B, 



X 



R 



1* --- 



55.3 



55-0 



53-68 



44-44 



B, 



X 



W 



2. 



109-6 



107-0 



94-60 



88-88 



B, 



X 



C 



X 



430.6 



4330 



339-0 



266-7 



B, 



X 



w 



1 

 "li 



900.0 



900-0 



736-0 



800-0 



Table III shows, as might be expected, that the uncertainty 

 of results increases with the power of the objective. 

 Theoretically, columns 2 and 3 ought to be identical. Practi- 

 cally they are nearly so for low powers, but the difficulty of 

 taking exact measurement with high powers is very great. 

 The inaccuracies revealed in column 4 are due partly to the 

 fact that the formula is only approximate, but also because 

 the tube length is not 250 millimeters, and cannot possibly 

 have this value with the instrument employed. In column 5 

 erroneous values of F and f\ taken from the labeling, so 

 greatly increase the errors of column 4 as to make the meas- 

 urements worthless. Yet these are the results of calculation as 

 commonly applied to the data furnished by the manufacturers 



In using the camera lucida the difficulty increases when the 

 higher powers are employed, just as much as in applying Cross's 

 formula. Under any circumstances, therefore, a wide margin 

 of uncertainty exists in estimating the magnification attained 

 with objectives of high power. Although the figures given 

 are in each case the mean of many measurements, the remark- 

 able agreement in the two results attained with the yVth is 

 doubtless to some extent accidental. With medium and lower 

 powers it is shown by comparison of columns 2 and 3 that 

 results about equal in value to those with the camera lucida are 

 had by taking the product of the separate magnifications due 

 to objective and eye-piece. And formulas (6), (7) and (13) show 

 that this product may be expressed as 



M 



(D + F)(T-/) 

 Ff 



(14). 



