[5] For further information, I must refer my readers to Parkinson’s “Treatise on Optics;” Herschel’s “Familiar Lectures on Light;” “Cyclopædia Britannica;” Everett’s translation of Deschanel’s “Physics;” and Nägeli and Schwendener’s “Theory and Practice of the Microscope,” translated by Frank Crisp, LL.D.
[6] The cornea of the eye is not so entirely the simple transparent structure as it at first sight may appear to be. It is composed of several layers, the most important of which is the nerve layer, consisting of innumerable ganglionic stellate plexus of cells held together by a network, as seen in [Fig. 21], a small section stained by chloride of gold, and magnified 300 diameters. Beneath the nucleated nerve cells is a second layer of stellate cells, varying a little in their form. These nerve and stellate cells serve the purpose of maintaining the cornea in health, and must play a significant part in the dioptric system.
[7] The standard condition of perfect vision is termed emmetropia.
[8] Landolt; “The Accommodation and Refraction of the Eye,” 1886.
[9] µ = ·001 of a millimetre. This measurement is now universally employed in microscopy.
[10] Diffraction effects may be observed without a microscope, indeed, the more striking are seen in connection with telescopic vision. A beautiful series of phenomena in illustration of the diffraction of light may be produced as follows: Draw on a large sheet of paper a series of geometrical figures, arranged at equal distances in a circle. A collodion photographic picture of these being taken, a series of small transparent apertures in the elsewhere opaque film will result. This film is then mounted, so that it may be in turn brought before the centre of a small hand telescope, previously adjusted to view an image of the sun. In this way we have an apparatus of the most compact form, and by means of which a series of fifty or more phenomena may be brought into view in a few minutes. These pictures being very small (occupying on an average area one-tenth of an inch in diameter), inaccuracies of surface and substance of the glass may be neglected. A film of Canada balsam with which the glass is cemented over the picture produces no disturbance. There is a manifest advantage in the figures being small, as the size of the image is in inverse proportion to the size of the aperture.
[11] Carpenter, “The Microscope,” p. 65, 1891.
[12] “Phil. Mag.,” viii., p. 167 (1896).
[13] Professor Stokes wrote me in the following flattering terms:—“What you have submitted to me on the subject of apertures is so sound, clear, and succinct, that I have nothing to add to it. The method adapted as you have explained respecting the immersion system, I consider to be perfectly satisfactory.” Subsequently, and at my request, Sir George Stokes contributed a valuable paper on the subject to the “Transactions of the Royal Microscopical Society,” 1876, on “The Theoretical Limit of Aperture.”
[14] “On the Estimation of Aperture in the Microscope,” “Journal of the Royal Microscopical Society,” series ii. vol. i.; “Notes on Aperture, Microscopic Vision, and the Value of Wide-angled Immersion Objectives,” 1881.