ZOOLOGY AND BOTANY, MICKOSCOPY, ETC 



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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 



Fig. 18. 



borders of the separate partial images should not show any other 

 tints than a very narrow edging of pure green, rose, or violet of the 

 secondary colours of a spectrum. Spherical aberration is revealed, 

 when, with the best focussing, the clear lines 

 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 ap- 

 proximately any objective is recommended by 

 Professor Abbe, as it is applicable to all instru- 

 ments 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 peri- 

 pheral edge of the field. As soon as this is done, replace the 

 eye-piece, and examine afresh the object plate loithout altering tJie 

 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 neces- 

 sary to get better definition, and colours confuse the images. 



A test image of this kind at once lays bare in all particulars the 

 whole state of correction of the Microscope ; it being of course 

 assumed that the observer knows how to observe and what to look 

 for. 



With the aid which theory offers to the diagnosis of the various 



