156 DR. C. ZEISS ON THE METHOD OF USING ABBE's TEST-PLATE. 



racter of the colour bands can be easily estimated. Differences in 

 the thickness of the cover-glass within the ordinary limits are 

 scarcely noticeable with such objectives. It is of fundamental im- 

 portance, in employing the test as above described, to have brilliant 

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



When, from practice, the eye has learnt to recognise the finer 

 differences in the quality of the contour images, this method of in- 

 vestigation gives very trustworthy results, and differences in thick- 

 ness of cover glasses of OOl mm. to O02 mm. can be recognised 

 with objectives of 2 mm. or 3 mm. focus. With oblique illumina- 

 tion the light must always be thrown perpendicularly to the 

 direction of the lines. 



The quality of the image outside the axis has no bearing on 

 spherical and chromatic correction in the strict sense of the term. 

 Indistinctness of the contours towards the borders of the field of 

 vision arise, as, of rule, from unequal magnification of the different 

 zones of the objective ; colour bands in the peripheral portion (witli 

 good colour correction in the middle) are caused by the 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 the aperture is considerable. 



