ZOOLOGY AND BOTANY, MICROSCOPY', KTC. 625 



conveniently, by use of a mechanical substage with rack-and-pinion 

 movement, by means of which the iris can be set centrally or escentric 

 at will. When spherical aberration is present in an objective, the 

 different zones focus at a different level, and consequently if the light is 

 changed from central to oblique without changing anything else, the lines 

 of the test-plafce will go out of focus if spherical aberration is present, 

 and the latter will thus be immediately detected. 



It was pointed out in this connection, that on the same principle 

 an extremely sensitive test for spherical aberration might be obtained by 

 using a condenser stop of such size and so decentred that it just 

 reached from the centre to the margin of the objective, and then 

 noticing whether the two edges of the test-plate line running across 

 the centre of the field were sharply in focus simultaneously. 



It is said that by this method of observation the correct tube-length 

 of a high-power objective can be determined within one or at most a few 

 millimetres. 



In a second Microscope the same excentric stop was employed for 

 testing the chromatic correction of an objective. The aperture in the 

 substage should in this case be quite small, so as to test a very narrow 

 zone of the objective at one time. Under these conditions, the edges of 

 the lines of the test-plate show complementary colours, which in the case 

 of a perfectly corrected modern objective should be apple-green and 

 purple or claret-colour, no matter what zone of the objective other than 

 the central one is tested. 



The achromatic objectives of the older type show a continual 

 change of the secondary colour-tints when the stop is moved from the 

 centre to the edge of the aperture. They usually show a bluish-green, 

 or even blue, instead of the apple-green near the centre of the aperture, 

 and a yellow, or even orange, instead of the apple-green in the marginal 

 zone. 



The third of the Microscopes in this section was fitted with a badly 

 centred objective, the result being that when the light had been care- 

 fully centred and adjusted, unsymmetrical colour fringes and fogginess 

 became apparent on the edges of the lines of the test-plate. One-sided 

 defects of this kind with carefully centred illumination are nearly always 

 due to centring defects in the objective, but may sometimes be caused 

 by the stage not being accurately square to the optical axis, or to a badly 

 mounted object having its cover-glass similarly out of square to the 

 optical axis. 



The fourth of these Microscopes showed the importance of either 

 using the correct thickness of cover-glass or else compensating the 

 effect of change in this respect by altering the tube-length, or by 

 using a correction collar. The 4-mm. Holos objective had been correctly 

 adjusted to the thinnest cover-glass on the Abbe test-plate. By moving 

 the mechanical stage so as to bring the adjacent thickest cover-glass 

 under the objective, the excellent image obtained in the first case was 

 immediately replaced by a hopelessly bad image caused by the change of 

 cover-glass thickness. 



In an adjoining room, the AYatson-Conrady apparatus for photo- 

 micrography had been set up, and its distinctive features were fully 

 explained and the method of using it demonstrated. 



