Nachet exhibited at the Antwerp Exhibition a fine 1/10-inch oil-immersion, which was highly praised by the jurors.
It is necessary, to make the fact perfectly clear, that dry and immersion lenses having the same angular aperture have also a similar defining power. The pencil of rays, however, differs in intensity and density as the rays emerging from the cover-glass of the object into air are very considerably deflected, and the cone suffers a corresponding loss of brightness. On this important point, then, I believe it will prove of value to interpolate a clear and full exposition of the change brought about by the cover-glass.
It is not difficult, then, to perceive the importance of Amici’s discovery as to the value of a drop of water inserted between the object and the objective, and it now seems somewhat surprising it should have been so long neglected by opticians, since it is at once seen to diminish the reflection which takes place in the incidence of oblique light. The film of water not only gives increased aperture, but also greater cleanness and sharpness to the image. The film, then, as already shown, collects the straying away of peripheral rays of light, and sends them on to the eye-piece, and greatly assists in rendering the image more perfect, and materially aids in the removal of residuary secondary aberrations; while with air, or dry objectives, a certain amount of aberration takes place, sufficient to affect the pencils on their passage from the radiant to the medium of the front lens, adding a considerable ratio to the total spherical aberration with the objective, which, in the case of wide angles, increases disproportionately from the axis outwards. This can only be corrected by a rough method of balancing; that is, by introducing an excess of opposite aberration in the posterior lens. An uncorrected residuum, rapidly increasing with larger apertures, is then left, and this appears in the image amplified by the total power of the objective, so that with a non-homogeneous medium there is a maximum angular aperture which cannot be surpassed without undergoing a perceptible loss of definition, provided working distance is required. If we abolish the anterior aberration for all colours, by an immersion fluid which is equal to cover-glass in refractive and dispersive power, the difficulty is at once overcome. If, for instance, we have an objective of 140° in glass (= 1·25 N.A.) and water as the immersion fluid, the aberration in front would affect a pencil of 140°. Substituting a homogeneous medium, the same pencil, contracted to the equivalent angle in that medium of 112°, will be admitted to the front lens without any aberration, and may be made to emerge from the curved surface also without any disturbing aberration, but contracted to an angle varying from 70° to 90°. The first considerable spherical aberration of the pencil then occurs at the anterior surface of the _second_ lens, where the maximum obliquity of the rays is already considerably diminished.