ern achromatic microscope, I venture to say that photography applied
to this instrument will be of no farther use than as _an assistant to the
draughtsman_. A reference to the plates alluded to will show how
incompetent it is to produce _pictures_ of microscopic objects: any one who
has seen these objects under a good instrument will acknowledge that these
specimens give but a very faint idea of what the microscope actually
exhibits.

It is unfortunately the case, that the more perfect the instrument, the
less adapted it is for producing photographic pictures; for, in those of
the latest construction, the aperture of the object-glasses is carried to
such an extreme, that the observer is obliged to keep his hand continually
on the fine adjustment, in order to accommodate the focus to the different
_planes_ in which different parts of the object lie. This is the case even
with so low a power as the half-inch object-glasses, those of Messrs.
Powell and Lealand being of the enormous aperture of 65 deg.; and if this is
the case while looking through the instrument when this disadvantage is
somewhat counteracted by the power which the eye has, to a certain degree,
of adjusting itself to the object under observation, how much more
inconvenient will it be found in endeavouring to focus the whole object at
once on the ground glass plate, where such an accommodating power no longer
exists. The smaller the aperture of the object-glasses, in reason, the
better they will be adapted for photographic purposes.

Again, another peculiarity of the object-glasses of the achromatic
microscope gives rise to a farther difficulty; they are over-corrected for
colour, the spectrum is reversed, or the violet rays are projected beyond
the red: this is in order to meet the requirements of the eye-piece. But
with the photographic apparatus the eye-piece is not used, so that, after
the object has been brought visually into focus in the camera, a farther
adjustment is necessary, in order to focus for the actinic rays, which
reside in the violet end of the spectrum. This is effected by withdrawing
the object-glass a little from the object, in which operation there is no
guide but experience; moreover, the amount of withdrawal differs with each
object-glass.

However, the inconvenience caused by this over-chromatic correction may, I
think, be remedied by the use of the achromatic condenser in the place of
an object-glass; that kind of condenser, at le