282 Transactions of the Society. 



magnifying power of the system the magnifying power of a com- 

 ponent optical element. 



Example I. — Where we desire to measure the magnifying power of a 

 pocket lens, we view through the lens in question a millimetre scale, dis- 

 posing this last at such distance from the lens as to give the largest possible 

 erect image. Holding the lens in position, we now place the eikonometer 

 between the eye and the lens and read off the number of divisions of its scale, 

 which are covered by one millimetre division on the object scale. The munber 

 of divisions covered corresponds to the magnifying power. 



Example 2. — Where we desire to measure the magnification of an objective, 

 we place a micrometrical ruling upon the stage, remove the ocular from the 

 Microscope tube, and focus down with the objective until we obtain the 

 largest possible erect image of the ruling. We now place a slip of glass over 

 the upper open end of the Microscope tube, and place upon the top of this 

 the eikonometer, and then read off the value of the divisions of the stage 

 micrometer upon the measuring scale. 



Example 3. — Where, knowing the magnifying power of the ocular, we 

 desire to measure that of the objective by the indirect method, we measure 

 by means of the eikonometer the total magnifying power of the Microscope, 

 and arrive at the magnifying power of the objective, taken separately, by 

 dividing the total magnification by the magnification which is referable to 

 the eye-piece component. 



Measurement of the Magnification of the Microscope by 

 the separate determination of the magnifying power 

 of the Objective and the Ocular. 



The possibility of arriving at the magnifying power of the 

 Microscope by the measurement of that of the optical components 

 of the Microscope taken separately, has already been adverted to in 

 the introductory section. In exploiting this principle of measure- 

 ment, we may leave altogether out of account what I shall venture 

 to call the " optical anatomy " of the Microscope. We may, in 

 other words, ignore the fact that the objective works in com- 

 bination with the field-lens of the ocular, and that the eye-lens of 

 the ocular works in combination with the optical system of the 

 eye ; and we may assume instead that the objective works in con- 

 junction with the optical system of the eye, and that the field lens 

 works in conjunction with the eye-lens. This will, if I may, for 

 the purpose of exposition, resort to a rough analogy from human 

 anatomy, be equivalent to taking together for purposes of measure- 

 ment, (a) the legs and the head and neck, and (b) the chest and 

 abdomen, instead of taking together, in the proper anatomical 

 order, (a) the legs and lower half of the trunk, and (b) the upper 

 half of the trunk and head and neck. 



We have in the previous section illustrated the method of 

 measuring the magnifying power of the objective working as a 

 doublet with the optical system of the eye. Here we may deal 

 with the method of measuring the magnifying power of the ocular 

 working as an independent optical element. 



