MEASUREMENT OF MAGNIFYING POWERS. 309 



B. — Measure with the stage micrometer tlie actual diameter 

 of the field. 



Then A divided by B is the magnifying power. 



It is to be noted, however, that the apparent diameter of the 

 field, as projected by the camera and measm'ed on the flat, is, for 

 the reason before stated, excessive, and a deduction must be 

 made, according to the rule to be stated farther on, to com- 

 pensate for the uneven magnification. 



Example. — The apparent field-diameter, as measured on the 

 flat, is 5 inches, or 127 mm. We deduct 3 millimetres, making 

 the eft'ective diameter 124 mm. The actual diameter of the 

 field, as shown by the stage micrometer, is 0*62 mm. Then 124-i- 

 0*62 = 200, which is the magnifying power. 



The diameter of the magnified field of view depends solely on 

 the eye-piece diaphragm ; it is therefore a constant quantity, 

 which when once measured may be recorded for future reference. 

 If preferred it may be scratched on the eye-piece tube. 



As soon as all the eye-piece fields have been measured and 

 recorded there is no further need for the camera, or for any other 

 accessory than the stage micrometer ; the microscope is used in 

 its ordinary position, and the power of each eye- piece tested in 

 combination ^dth each objective till the table is completed. The 

 same simple process enables us to find at once what power we 

 are using at any time when, owing to variation of the tube-length, 

 our standard scale is not applicable ; a useful feature in these 

 days when the common use of such imperfect appliances as 

 fourths and sixths without cover-correction must, one would 

 think, lead to a continual pushing in and out of draw-tubes. 



I may here note a few points which require attention when 

 making our measurements, if reasonable accuracy is to be 

 obtained. We must see that all draw-tubes are pushed home, 

 unless it is preferred that the magnifications should be taken 

 with a 10-inch tube-length, vv^hen the tube must be set accord- 

 ingly. The objective collar must be looked to, as its position 

 appreciably affects the power. I have found that with a J-inch 



the ends slightly apart, bring them up from the lower side of the eye- 

 piece at an angle of 45 degrees, so that a thin cover- glass laid upon 

 them will reflect the image of the field, apparently upon the jjaper. I 

 have tried this expedient, which only took about three minutes to 

 carry out, and it was quite as efficient as the ordinary Beale reflector. 



