Bayswater.
_Photography and the Microscope_ (Vol. vii., p. 507.).--I beg to inform
your correspondents R. I. F. and J., that in Number 3. of the _Quarterly
Journal of Microscopical Science_ (Highley, Fleet Street) they will find
three papers containing more or less information on the subject of their
Query; and a plate, exhibiting two positive photographs from collodion
negatives, in the same number, will give a good idea of what they may
expect to attain in this branch of the art.
Practically, I know nothing of photography; but, from my acquaintance with
the modern 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 least, which is supplied by the
_first_ microscopic makers. I cannot help thinking that this substitution
will prove of some service; for, in the first place, the power of the
condenser is generally equal to that of a quarter of an inch object-glass,
which is perhaps the most generally useful of all the powers; and again,
its aperture is, I think, not usually so great as that which an
object-glass of the same power would have; and, moreover, as to correction,
though it is slightly spherically under-corrected to accommodate the
plate-glass under the object, yet the chromatic correction is _perfect_.
The condenser is easily detached from its "fittings," and its application
to the camera would be as simple as that of an ordinary object-glass.
However, my conviction remains that, in spite of all that perseverance and
science can accomplish, it never will be in the power of the photographer
to produce a picture of an object under the microscope, _equally distinct
in all its parts_; and unless his art can effect this, I need scarcely say
that his best productions can be but useful auxiliaries to the draughtsman.
I see by an advertisement that the Messrs. Highley supply everything that
is necessary for the application of photography to the microscope.