74 Modern Microscopy 



sharp throughout, the corrections for spherical aberration 

 are eminently satisfactory ; but should a difference of focus 

 occur, to avoid all chances of erroneous deduction the 

 other discs should be examined to insure that the proper 

 thickness of cover is being used ; also the tube-length might 

 be varied, and if after these precautions it is still found 

 that there is a difference of focus over the intermediate 

 position of the diaphragm, the existence of a spherical zone 

 is at once demonstrated. This process enables the best 

 tube- length and thickness of cover for the objective to be 

 discovered with accuracy. 



CHROMATIC CORRECTION. Tests for this should be made 

 in the same manner as for the spherical correction. 

 Under the same conditions an apochromatic should show 

 practically no colour, or, at the most, barely distinguish- 

 able traces of tertiary tints. Semi-apochromatics, or 

 lenses of fine correction, will show narrow bands of 

 pale green (apple-green) on one side, and faint purple 

 (or claret) on the other side, of each line, and the same 

 colours or tints should appear whether the diaphragm 

 be used centrally or obliquely, the width of the colour 

 bands only changing; further, good definition should be 

 yielded under all circumstances. Ordinary lenses will 

 generally show the best colour correction for some inter- 

 mediate zone. If they exhibit broad bands of primary 

 colour yellow or blue with very oblique light, the defini- 

 tion will be found to be bad. 



CURVATURE OF FIELD. There is one point concerning 

 which some slight difficulty may arise in connection with 

 the curvature of the field. Absence of flatness of field, 

 which is inherent in the construction of all latter-day 

 objectives, and particularly so in those of high power and 

 large aperture, is not regarded as a fault, but coma in an 

 objective at first sight gives the same appearance ; but the 

 difference is this: when the latter fault exists only the 

 central line or those nearest to it can be focused sharply, 

 those towards the margin remaining indistinct or ill-defined 



