80 PBACTICAL PHOTO-MICROGBAPHY 



dry lenses), the most perfect correction that corresponds 

 to a particular thickness can be estimated with exactness. 

 The test-plate consists of six discs of thin microscopic cover- 

 glass of accurately determined thickness (0-09 mm. to 0-24 mm.) 

 cemented side by side. Each of these discs is silvered on its 

 lower surface, and has a series of lines ruled on it, so that 

 it is in effect a coarse grating of alternating silvered lines 

 and clear spaces. The lines are usually not quite opaque, 

 but transmit sufficient light to enable the grain of the silver 

 deposit to be seen. On examination under the microscope 

 with a low power, therefore, a large number of these lines come 

 into the field, but with high powers possibly only two or three 

 lines will be shown : in either case the object is so thin and 

 each line has such a number of prominences on either edge 

 that it affords an extremely valuable, and comparatively easy, 

 method of at once determining some of the qualities of the 

 objective under examination. 



Under axial illumination and when the image is exactly in 

 focus the edges of the lines should appear perfectly sharp with no 

 apparent doubling ; neither should there be any but the very 

 slightest indication of colour fringes with achromatic lenses. 



The critical test is to use oblique light, and the light should 

 fall in a direction at right angles to that of the lines ruled 

 on the test-plate. Each objective should be tested first by 

 oblique, and then by direct axial light, and on changing from 

 one method to the other no alteration in the focus of the 

 objective should be necessary that is, the object having been 

 focussed by oblique light, its image should appear to be equally 

 well defined when illuminated by axial light. This test indicates 

 the degree of correction of the lens for spherical aberration. 



To test for chromatic correction the same methods of 

 illumination should be arranged for. An achromatic lens 

 should show only narrow fringes of colour on each side of the 

 lines on the test-plate, and these should be the complementary 

 colours of the secondary spectrum, on one side greenish, and 

 on the other a reddish violet. With apochromatic objectives 

 of the highest quality particularly those made by Messrs. 

 Zeiss there is little or no evidence of any coloured margin to 

 the lines under any conditions of illumination. 



In working with a test-plate the light used should be fairly 



