The tercentenary of the compound microscope : an inaugural address delivered November 7, 1890 to the Scottish Microscopical Society / by W. Rutherford.

  • William Rutherford
Date:
1891
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Licence: Public Domain Mark

Credit: The tercentenary of the compound microscope : an inaugural address delivered November 7, 1890 to the Scottish Microscopical Society / by W. Rutherford. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

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    of achromatic lenses appears to have been given by Joseph Jackson Lister L a paper pnblished in 1830,^ in which he ^.owed that Sellio-ue’s method of superposition of achromatic doublets in obiertive was capable of yielding results not hitherto attanied because the key to them had not previously been found. Lister laid due stress on the principle that an objective must have ® of aperture to give a brilliant and distinct image. He showed that the ^marginal rays of a luminous pencil are those which specia ly serve to resolve fine closely adjacent lines such as those on the scales of Lepidoptera, as may be proved by the fact that some of the most difficult of these lines are best seen when only the marginal rays ate employed and the central rays stopped out. He stated that the “great requisite for the object-glass of a compound microscope is a large focal pencil free from aberration; that the field should be flat and well defined throughout, and that the light admitted should as far as possible be only that necessary for the fomiation of the picture, and that it should not be intercepted or diffused over the field by too many reflections.” He said that the prominent obstacle to obtaining a sufficient pencil of light for high powers by one object- crlass of large aperture and deep curves is, that the correction for spherical aberration by the concave lens is proportionally greater lor the marginal than for the central rays, so that there is over-correc- tion of the marginal rays and the image consequently rendered indistinct, and at the same time coloured. It, therefore, becomes necessary to cut off the marginal rays, and so diminish the aperture of the lens. Lister was the first to show that by superposition of achromatic doublets the effective aperture of the series of lenses can be widened by a precise adjustment of the distance between them, and by accurately centering them around the optic axis. In combin- ing several lenses together he says “ it is often convenient to trans- mit an under-corrected pencil from the front glass, and to counteract its error by overcorrection in the middle one” (p. 199). These apparently simple indications were what he termed the “key” to the improvement of achromatic objectives. Lister’s suggestions were promptly acted on by London opticians, more especially by Smith and Beck, Andrew Ross, and Hugh Powell, whose ingenuity and skill rapidly raised English microscopes to the first rank for optical as well as mechanical excellence. No better testimony could be given to Lister’s acuteness in 1830, than that furnished by Professor Abbe^ in 1879 in his paper on the correction of spherical and chromatic aberration, where he shows the great importance of the relative distance between the lenses of an objective, and the excellent results attainable when an under-corrected pencil of light is transmitted to over-corrrected lenses placed at a suitable dis- tance above the under-corrected front lens, the compensating power 1 “On some rro]in.rtics in Acliioinatic Oi'ject Glasse.s applicable to the Im- provement of the Microscope,” by J. J- Lister, Phil. Trans., London, 1830, part i., p. 187. “ Abbe “On New Methods for Improving Spherical Correction apjilied to the construction of Wide-Angled Object-Glasses,” Jour. Hoy. Mic. Soc., 1879, p. 812.