lO ON OPTICAL QUALITY OF MICROSCOPE OBJECTIVES. 



inch, the ruled side is then cemented with balsam on a polished 

 glass slip ; several such prepared glasses being cemented side by 

 side on the same slip. 



A. perfectly corrected objective, tested with the test object, and 

 by the mode of illumination above described, ought to show over 

 the middle of the field a clearly defined image of the groups of 

 lines under examination without any alteration of focus, and the 

 coloured borders of the separate partial images, should not shew 

 any other tints than a very narrow edging of pure green, rose or 

 violet of the secondary colours of a spectrum. Spherical aber- 

 ration is revealed, when, with the best focussing, the clear hnes 

 appear as if immersed in the middle of a broader foggy streak, or when 

 two images, more or less, overlapping each other, merge on altering 

 the focus, into one image, somewhat broader and more misty. 



A short and ready method of testing approximately any 

 objective is recommended by Professor Abbe, as it is applicable 

 to all instruments without requiring any apparatus except the 

 test object already described. 



This may be briefly explained as follows : 



First, focus the test plate with central illuminating rays, then 

 withdraw the eye-piece, and turn aside the mirror so as to give 

 the utmost obliquity of illumination, which the objective under 

 trial will admit of. This will be best determined by looking 

 down the tube of the microscope whilst moving the mirror, 

 and observing when the elliptic image of light reflected from it, 

 reaches the peripheral edge of the field. As soon as this is done, 

 replace the eyepiece, and examine afresh the object plate without 

 altering the focus. If the objective be perfectly corrected, the 

 groups of lines will be seen with as sharply defined edges as before 

 and the colours of the edges must, as before, appear only as those 

 of the secondary spectrum in narrow and pure outline. Defective 

 correction is revealed when this sharp definition fails, and the lines 

 appear misty and overspread with colour, or when an alteration of 

 focus is necessary to get better definition, and colours confuse the 

 images. 



