Draw-tubes versus Deep Eye- pieces. ] 29 
different results above noted do not depend upon any defect in the 
correction of the spherical or chromatic aberrations of the objective 
combination, but are produced by the variable amount of inter- 
ference of the rays of light in the two given conditions, the fringes 
of interference becoming larger and more injurious to the image 
as the tube is lengthened and the luminous cone becomes more 
acute. 
It is well known that the capacity of an objective to display the 
details of a surface depends largely upon its angle of aperture ; the 
larger this angle, the finer the objective. We make the single 
remark, that the magnitude of the angle of aperture has reference 
only to those rays that enter into the formation of the image. If 
the aberrations are not corrected through the whole extent of the 
angle, those rays that find entrance through the uncorrected por- 
tions, unless for the formation of the image only, serve to flood the 
field with light, in which the image is lost, drowned. The larger 
the angle of aperture, the larger the surface of the back lens of the 
objective. The circle formed by this back lens, which receives all 
the light introduced into the objective, is the base of a cone, of 
which the summit is found in the conjugate focus of the objective, 
at the point where is formed the image seen by the eye-piece. In 
objectives of high power the base of this cone, as compared with the 
distance separating it from the eye-piece, is very small; the cone is 
very pointed, and the rays intersecting one another at its summit 
make an angle necessarily very acute, and more acute in proportion 
as the summit is removed from the objective. 
We know that rays of light crossing one another at very acute 
angles do give rise to this phenomenon of interference, and that 
the fringes are larger in proportion as the angle of intersection is 
diminished. Applying this principle to the problem under consi- 
deration, we find that the rays passing near the centre of the objective 
must give rise to this phenomenon, and that as the rays pass from 
the centre the fringes of interference will be less, and the definition 
consequently finer and sharper. This improvement in the defini- 
tion will continue until the rays approach the border of the lens, 
when, from imperfect corrections of aberrations, or too great an 
angle of incidence, they no longer enter into the formation of the 
image. 
We may say, for the sake of simplicity, that the extreme rays 
nearest the borders of the objective form a cone, the base of which 
is upon the objective and the apex in the focus of the eye-piece. In 
proportion as we increase the length of the tube of the microscope, 
or, what is the same thing, the distance between the objective and 
eye-piece, the luminous cone becomes more pointed, for the base of 
it is unaltered, while the height of it is increased. The angle of 
intersection of the rays at the summit is diminished, while at the 
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