116 THE AMERICAN MONTHLY [Apr 



10 iu. from the hole in the eye-cap, and if the eyepiece 

 has no eye-cap, then the screen should be 10^ inches from 

 the eye-lens. 



When the power of the eyepiece is once known, the 

 focal length of any other objective may be found, with- 

 out taking the trouble to project the image on the screen, 

 by first determining its combined magnifying power by 

 means of the camera and the scale, as above. For ex- 

 ample, the combined magnifying power of Zeiss AA, 

 with the same eye-piece and tube-length, was found to be 

 80 diameters. Next find the initial magnifying power of 

 the AA by dividing the combined power by the power of 

 the eyepiece, thus 80 divided by 5.05 equals 15-8. Now 

 divide 10 by this initial magnifying power, 15.8 and we 

 have 2-3 (nearly), the focal length of the AA. All nii- 

 croscopists should measure in the above manner the focal 

 length of at least one of their object-glasses, preferably 

 a 1-4 in. and determine for themselves the magnifying 

 power of their various eyepieces, instead of relying on 

 the values assigned to them in the makers' catalogues. 



Another Way to Get Focal Length. 



By j. g. p. v. 



Put a stage micrometer on the stage, and focus the 

 glass on it ; then turn the microscope horizontally, and 

 pin a piece of paper vertically in line with the axis and 

 parallel to the stage. It is most convenient to fasten 

 this paper on the wall. Remove the eyepiece, and focus 

 again till the image of the micrometer is sharp on the 

 paper. This will only be the case with the centre lines. 

 Measure their size carefully, and measure the distance 

 between the micrometer and the image. Let I) be this 

 distance, F the required focus, and M the magnification ; 

 then F is equal to D multiplied by M, and the product 

 divided by the square of M plus one. If the distance is 

 a fair length — about 8 ft. — the distance divided by the 



