168 



TESTING THE MIC110SCOPE. 



IV. 



TESTING THE FLATNESS OF THE FIELD OF VIEW. 



As the traditional expression, " curvature of the field of view," 

 includes, as we have above shown, two independent notions, viz., 

 (1) the distortion of the image, and (2) the curvature of the image- 

 surface, they must be considered separately in the testing of the 

 Microscope. As regards the distortion of the image, its extent 

 may be determined by viewing a rectangular network (e.g., a glass 

 micrometer divided into rectangular spaces) under the Microscope. 

 If the enlargement increases or decreases from within outwards, the 

 image will appear as in Fig. 24 or Fig. 23 respectively ; but if no 

 distortion takes place, the meshes of the image will coincide with 

 those of the object. From the curvature of the lines in Figs. 23 

 and 24 it also follows that a single straight line, if brought near to 

 the margin of the field of view, will form an excellent test-object. 



We may test the behaviour of the images, which the objective 

 forms alone, or with the aid of the field-lens, as follows: A 

 cover-glass with a straight edge is placed upon the diaphragm in 

 the eye-piece setting (the eye-lens being removed) so that the edge 

 appears in the circular diaphragm as a chord of arc (m n in Fig. 

 98). The real image of another straight line, which is viewed as 



FIG. 98. FIG. 99. FIG. 100. 



an object, is made to coincide with this chord precisely as the ad- 

 justment of a particular division-line to the margin of an object is 

 effected in micrometric measurements. If a complete coincidence 

 takes place whether or not they appear straight or curved in 

 the virtual image then the real image is free from distortion ; 

 in all other cases it is distorted. 



