MAGNIFYING POWER. 



179 



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The methods which are employed for the determination of the 

 magnification, all amount to the same thing, viz., the projection of 

 the image of an object of known magnitude to the conventional 

 distance of 25 cm., and the direct comparison of its diameter with 

 that of the image-forming object. The projection may be made by 

 means of Scemmerrmg's mirror or a reflecting prism in which the 

 reflecting surface must be situ- 

 ated so that the second focal 

 point of the Microscope shall 

 coincide with the focal point 

 of the rays in the eye of the 

 observer. Let N* (Fig. 103) be 

 the last refracting surface (the 

 plane surface of the eye-lens), 

 F* the second focal point, and 

 s a reflecting surface inclined 

 45 C ; the path of the rays will 

 thus be altered as if the optic 

 axis (0 x) of the system were 

 horizontal, and the focal point 

 brought to (F)*. The eye must 

 therefore be placed at this latter point, if the results of the 

 measurements are to be exact and capable of comparison. Its 

 position corresponds very nearly to that in which the emergent 

 pencils form the smallest circle of light on a screen of ground 

 glass placed at right angles to the axis. It is, moreover, evident 

 that the plane upon which the image is projected (P P, Fig. 

 103) must form a right angle with the direction of the optic 

 axis as altered by the reflexion. 



In selecting an object, it is essential that its margin, or rather 

 the points whose distance is required, be clearly seen in the image. 

 A glass micrometer is convenient for this purpose ; but if the lines 

 appear very indistinct, as generally occurs with the higher magni- 

 fications, other objects with sharp margins (for instance, air-bubbles, 

 globules of mercury, crystals, &c., whose diameter can be micro- 

 metrically determined) should be employed. The accuracy of the 

 determination is dependent in every case upon the exactness of 

 the divisions of the micrometer. 



The measurement of the virtual image on the plane of projection 

 is most easily effected by means of drawing-compasses, which can 



FIG. 103. 



