194 Extracts from Mr. H. E. Fripp's Translation of 
the part taken by each constituent element of a compound system 
of lenses in the joint performance of the whole. When then we 
define the function of the objective to be the production of a real 
image, and the function of the eye-piece the amplification of this 
image, — such explanation does not by any means reach the essential 
principle of action of the compound microscope. This is obvious at 
once when we consider that by such a definition the combination 
of objective and eye-piece is made only to indicate magnifying 
power, whereas on the contrary the remarkable superiority of 
compound over simple microscope consists in the quality of Us 
performance. By the objective an image is formed and spread out 
in what is an almost perfect accordance with the laws by which 
images of infinitely small elements of a surface are formed. By 
the eye-piece a displacement of focus is effected ; that is to say, a 
change of divergence of each separate pencil of light takes place till 
the divergence is almost imperceptible, and the pencils infinitely fine. 
The first step or act in the image-forming process consists, not 
in the production of a reversed image by the objective in front of 
or within the ocular, but rather in the production of a " virtual " 
image at an infinite distance with parallel rays. The second act 
comprises the last refraction through the posterior surface of the 
objective, and the several refractions taking place in the ocular 
by which the image is re-formed at the distance of clear vision with 
diverging visual angles. The first act answers plainly to the 
function of an ordinary " magnifying glass " ; while the second, 
taking all the changes comprised therein together, answers as 
obviously to the functions of the telescope (possessing only a small 
objective aperture) to which the virtual image formed by the first 
process serves as " 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 characteristic 
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 focussing 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 
