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

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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$illustration is sufficient to show what jonf^jon adopting the numerical aperture as an nide. o The numerical aperture of objectives may vary from as low cv W of 14, the aperture ot the h.ghest rtiAoniirnninfin oil-iniuiersion lens made by Zeiss. ^ ^ ^ in of the resolving power of lenses winch has taken pCe in recent years, would have been impossible but for the adoption of the immersion principle. _We are Alodena for the invention of immersion lenses in 1^40, oi s o y after To him belongs the credit of having been the first to aim at ‘the construction of ^ con- certain oils have a refractive index similar to that of glass, he coi striicted lenses to be immersed in them, so that the refraction a reflection at the upper surface of cover glass and lower suiface o obiective mioht practically be abolished. It was a similai ic^^a tin led Robert Hooke nearly two centuries before to fill the tube of the microscope with water. The Italian microscopists, the oil attack the surface of the lens, so that Amici was obliged to abandon it and make lenses for immersion in water. In makin^ lenses for oil immersion fifty years ago. Amici was because powerful objectives with a sufficiently large angle of apeituie to take advantage of the homogeneous immersion principle had not then been constructed. Amici exhibited his water-immersion lenses in Paris, and similar objectives were made there, but they were soon surpassed by the immersion systems made in this country by Powell and Lealand. The adoption of the immersion principle for high power objectives has permitted of their construction with a larger aperture, so that their resolving power is increased ; but lor very hio-h power lenses water has now given place to thickened cedar oil, having refractive index Pol2, which is so nearly that of crown glass (P53) that when interposed between the cover glass and lens it virtually forms with them a homogeneous optical system, in which there is practically no reflection or refraction at u]iper surface of cover and lower surface of lens; consequently illumination and de- finition are improved, and resolving power increased, because the oil permits of the lens being constructed with a larger aperture than is possible with a water lens. The highest numerical aperture of the oil lenses made under Abbe’s direction is 1'4, which is regarded by him as the highest useful aperture. He believes it impracticable to attempt any further increase of aperture, because of the difficulty of overcoming spherical aberration, which increases with the aperture. The first oil-immersion lens in the recent period of the microscope was constructed by Zeiss from a formula calculated by Abbe,—at the suggestion of Mr J. W. Steiihenson, treasurer of the Royal Micro- scopical Society. The celebrity so rajiidly attained by the firm of Zeiss, as practical opticians, is mainly due to their association with a skilled mathematician and master of optics in the jicrson of Pro- fessor Abbe. Tlie association of the mathematical theorist with the practical optician is always desirable, and in these times no optician need hope to accomplish anything remarkable unless he is himself$