58 IF". LeConte Stevens — Microscope Magnification. 



in this respect. In column 1 the capital letter arbitrarily stands 

 for the name of a maker, and the adjacent figures for the focal 

 length of the objective as labeled, in inches or fractions of an 

 inch, on the mounting. Column 2 gives the distance in milli- 

 meters between the stage and eye-piece micrometers, determined 

 by the length of the microscope body ; and column 3, the cor- 

 responding magnification attained. Column 1 gives the com- 

 puted focal length in millimeters, which in column 5 is 

 reduced to inches for the sake of comparison ; and column 6 

 gives roughly the percentage of error of the label. 



On examination of Table II it is seen that the errors of the 

 labels are more frequently positive than negative, or that 

 objectives are more frequently labeled too low in power than 

 too high ; and that the errors are unpardonably great in the 

 objectives of lowest power. It seems scarcely conceivable that 

 an error of 40 or 50 per cent could be made and deliberately 

 stamped on the mounting of an objective whose real focal 

 length is so easily found by experiment. It should be observed 

 that any error due to thickness of cover glass is negligible 

 when the focal length exceeds 20 mm . The stage micrometer 

 used in these experiments was uncovered ; and since the higher 

 powers are usually adjusted to give their best definition when 

 a definite thickness of cover glass is employed, this fact may 

 partly account for the negative errors found in the two highest 

 powers examined, although the adjustment of collar in these 

 measurements was for use without a cover glass. 



Having obtained the magnifying powers of objective and 

 eye-piece, their product is the total magnifying power of the 

 combination. If the equivalent focal length of the eye-piece 

 is definitely known, its magnifying power, m', is obtained by 

 applying formula (7). If the tube length, T, and focal length, 

 y, of the objective are known, its magnifying power, m, may 

 be accurately obtained. For, referring to fig. 1, 



a'b' T _ ± 1 1 1 ~ T/ 



m= — - = — -. But -p— + — = -. .-. Oc= rfr—„. Hence, 

 ab Oc Oc T / T-/ 



T 

 m = — - 1 (13) 



In formula (13), if f be very small in comparison with T, 

 the term — 1 may for all practical purposes be neglected. But 

 to do this involves serious error when objectives of low power 

 are employed. 



Table III shows the result of using eye-piece B 1 of Table I 

 successively in combination with five of the objectives of 

 Table II, the values of m! and m being taken from these two 

 tables. Column 2 gives the values thus calculated, while col- 

 umn 3 gives the corresponding results independently obtained 



