1899] MICROSCOPICAL JOURNAL. 115 



size of the image by that of the stage micrometer. 

 Measure accurately the distance from the screen to the 

 front lens of the objective. Divide the screen distance 

 by the magnifying power plus one. 



Example: A reputed quarter projects the image of .01 in. 

 of a stage micrometer on a white screen distant 38 in. 

 from the front lens. The magnified image measures 1.8 in. 

 The magnifying power is therefore 180. Add one to this 

 and divide 38 in. by 181 which gives .21, This shows that 

 the reputed quarter is nearly 1-5 in. in focus. Instead of 

 taking the trouble to divide the 38 by 181, it is shorter to 

 divide both the numerator and the denominator of the 

 fraction by the numerator, 38, which gives numerator 1, 

 denominator 4|. Having found the focal length of the 

 reputed quarter, the magnifying power of a Huyghenian 

 eyepiece can be determined in the following manner: — - 

 Place the microscope in a horizontal position, a microme- 

 ter on the stage, a camera — Wollaston's, Seal's, or one of 

 similar form — on the eyepiece, and a scale divided into 

 inches and tenths on the table, directly underneath the 

 camera and at a distance of 10 in. from the camera. Next 

 measure accurately the distance from the front lens of the 

 objective to the diaphragm in the eyepiece. Now sup- 

 pose that the magnified image of .01 in. of the stage mi- 

 crometer, as projected on the scale lying on the table, 

 covers 2.4 inches it is obvious that the combined magnify- 

 ing power of the objective and eyepiece, with the same 

 tube length as above measured, is 240 diameters. Now, 

 as we know the focal length of the objective is as above, 

 its initial magnifying power will be 47.5. 



Dividing, then the combined magnifying power by this 

 quantity, we find the magnifying power of the eyepiece, 

 thus 240 divided by 47.5 equals 5 05. 



If a camera is not at hand, the combined magnifying 

 power may be found by projection on to the screen used 

 above ; but the screen must be placed at a distance of 



