All lenses, however, should be tried by the opticians who sell them; and if they presented a specimen of their powers to a buyer, he could see in a moment what their capabilities were.
Weld Taylor.
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°; 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.