94 THE MICROSCOPICAL NEWS. 
ously to the functions of the telescope (possessing only a small 
objective aperture) to which the virtual image formed by the first 
process serves as an “object.” 
This interlocking of objective and ocular functions—presenting 
the combined effect of a magnifying glass and that of a telescope 
—must be laid down as the most general and correct character- 
istic of the principle upon which the compound microscope of the 
present day is constructed. 
From the foregoing remarks may be gathered a theory of aberra- 
tions, sound and strong enough to master the difficulties which the 
application of exceptionally large angles of aperture to microscope 
objectives has occasioned. 
It appears that the faults of image formation are separable into 
two distinct classes, one comprising faults of the focusing act 
(aberrations in the strictest sense), the other comprising faults of 
the amplifying function. To the first class belong those spherical 
and chromatic aberrations commonly studied ; in the second class 
must be placed a series of peculiar deviations of rays of light from 
their normal course, which arise from the circumstance that the 
separate rays of a homofocal beam occupying the aperture of the 
lens yield unequally magnified images, according as their inclination 
to the axis varies, and according also to the unequal refrangibility 
of the different colours—an inequality which obtains just as much 
whether the several partial images are compared with each other, or 
whether within the area of each image different positions in the 
field of vision are compared. 
This class of anomalies affects exclusively the constitution of 
the image outside the centre of the field. The perfection with 
which the rays unite in the central region, and therewith the 
maximum capacity of performance, depends on the contrary en- 
tirely on the real aberration spherical and chromatic, as commonly 
understood. : 
Chromatic aberrations, as they show themselves where a large 
angular aperture is used, do not depend alone on those differ- 
ences of focus which affect the image-forming beams as a whole ; 
but quite as much in an unavoidable inequality of coincidence of 
colours of variously inclined pencils of rays within the angle of 
aperture, which manifests itself in this, that an objective which 
is perfectly achromatic when direct illumination is used must be 
more or less over-corrected for use with oblique illumination. 
Although the first-mentioned ordinary form of colour dispersion 
(primary and secondary) may be entirely removed or rendered 
scarcely noticeable, the last-named source of chromatism cannot 
be counteracted or removed by any known material or any known 
technical treatment. 
Spherical aberration on a stricter examination of its causes re- 
