112 THE MICROSCOPICAL NEWS. 
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 show 
any other tints than a very narrow edging of pure green, rose, or 
violet of the secondary colours of aspectrum. 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 approximately 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 
peripheral edge of the field. As soon as this is done replace the 
eye-piece, and examine afresh the object plate w¢thout altering the 
focus. If the objective be perfectly corrected the groups of lines 
will be seen with as sharply defined edge 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 az alteration of focus is 
necessary 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 
aberrations, a comparison of the coloured borders of the separate 
partial images, and an examination of their lateral separation and 
their differences of level, as well in the middle as in the peri- 
pheral zones of the entire field, suffices for an accurate definition 
of the nature and amount of the several errors of correction, each 
of them appearing in its own primary form. ‘Therewith we also 
see that which arises from aberration, properly so called (faults of 
focussing function), clearly separated from such imperfections or 
anomalies as spring from mere differences of amplification between 
unequally converged and unequally refracted rays; and, more- 
over, we eliminate completely all influence of the ocular on the 
quality of the image. 
