Ch. IX] MAGNIFICATION OF OBJECTIVES AND OCULARS 307 



To find the magnification of any objective at this image distance 

 one can proceed as for the simple microscope, but the better method 

 is by the use of the ocular micrometer. Two micrometers of known 

 value are needed, — a stage micrometer and an ocular micrometer in 

 divisions of a millimeter. 



(A) Determination of the magnification with a Huygenian ocular 

 with fixed or movable scale (fig. 91). Remove the tield-lens and focus 

 the ocular micrometer lines by raising or lowering the eye-lens. Focus 

 the stage micrometer lines, and make the lines of the two micrometers 

 parallel. Make the lines of the two coincide. Suppose the image of 

 0.2 mm. on the stage micrometer covered 2mm. on the ocular mi- 

 cometer, the magnification in this case would be 2mm. divided by 

 0.2 mm. = 10. One could also use the filar micrometer as directed 

 in § 243. A positive ocular has the advantage that nothing needs to 

 be changed in it. 



(B) Effect of the field-lens. For getting the independent mag- 

 nification of the objective the field-lens of the Huygenian ocular must 

 be removed, but for determining the magnification of the object ive 

 when used with the field-lens in position as in ordinary observation, 

 reinsert the field-lens and determine the magnification of the com- 

 bined objective and the field-lens exactly as directed above. One 

 can tell in this way also how much the magnification of the ocular is 

 reduced by the field-lens. It is very marked (fig. 23-24, 183). 



(C) Effect of tube-length. The effect of tube-length on the magni- 

 fication of the objective is discussed in §236. The general law i> that 

 with a given lens or combination the more distant from the lens the 

 image is formed the greater is the magnification; therefore in every 

 case the conditions must all be made exactly alike if the results are 

 to be similar. This is easily proved by getting the magnification of 

 the objective on the ocular micrometer with a tube-length of 160 mm. 

 and then with 250 mm. If one has a projection microscope the dif- 

 ference is strikingly shown by using an objective alone and getting 

 the magnification at a screen distance of 1 meter and then at 2 meters. 

 The magnification will be almost exactly twice a- great at 2 meters 

 as at one meter. The same holds for the projected virtual image, a- 

 one can see by fig. 85 and 182. 



