388 OBJECTIVES, EYE-PIECES, THE APEETOMETER 



correction the edges of the lines in the centre of the field should 

 show only narrow colour-bands in the complementary colours of the 

 secondary spectrum, namely, on one side yellow-green to apple-green, 

 and on the other, violet to rose. The more perfect the correction of 

 the spherical aberration, the clearer this colour-band appears. 



To obtain obliquity of illumination extending to the marginal 

 zone of the objective, and a rapid interchange from oblique to 

 central light, Abbe's illuminating apparatus is manifestly defective 

 011 account of its spherical aberration. We want at least his 

 achromatic condenser. For the examination of ordinary immersion 

 objectives, the apertures of which are, as a rule, greater than 180 

 in arc (1*00 N.A.), and those homogeneous immersion objectives 

 which considerably exceed this, it will be necessary to bring the 

 under surface of the test-plate into contact with the upper lens 

 of the illuminator by means of cedar oil, even if water-immersion 

 objectives are used. We may add, as a matter of experience, that 

 having once centred the light and the condenser, we hold, with 

 deference to Dr. Abbe, that the light should on no account be 

 touched, which, to obtain obliquity, he advises by mirror changes. 

 We believe that this should be secured solely by the movement of the 

 diaphragm. 



For the examination of objectives of smaller aperture (less than 

 40 to 50)? we may obtain all the necessary data for the estimation 

 of the spherical and chromatic collections by placing the concave 

 mirror so far laterally that its edge is nearly in the line of the optic- 

 axis, the incident cone of rays then only filling one-half of the aper- 

 ture of the objective, by which means the sharpness of the outlines 

 and the character of the colour-bands can be easily estimated. 



It is of fundamental importance, in employing the test-plate, to 

 have brilliant illumination and to use an eye-piece of high power. 

 With oblique illumination the light must always be thrown perpen- 

 dicularly to the direction of the lines. 



When from practice the eye has learnt to recognise the finer 

 differences in the quality of the outlines of the image, this method 

 of investigation gives very trustworthy results. Differences in the 

 thickness of cover-glasses of O'Ol or 0'02 mm. can be recognised with 

 objectives of 2 or 3 mm. focus. The quality of the image outside 

 the axis is not dependent on spherical and chromatic correction in 

 the strict sense of the term. 



I inlistinctiiess of the outlines towards the borders of the field of 

 view arises, as a rule, from unequal magnification of the different 

 zones of the objective ; colour-bands in the peripheral portion (with 

 good colour-correction in the middle) are always caused by unequal 

 magnification of the different coloured images. Imperfections of this 

 kind, improperly called 'curvature of the field.' are shown to a 

 greater or less extent in the best objectives, when their aperture is 

 considerable. 



Testing an objective does not mean seeing the most delicate 

 points in an object ; it rather means the manner in which an object 

 of some si/e is defined. 



A test for low powers up to ^ of 80 or X.A. -li."") is an object on 



