On eye-pieces for telescopes and microscopes / [Cornelius Varley].

  • Varley, Cornelius, 1781-1873
Date:
[1835?]
Catalogue details

Licence: Public Domain Mark

Credit: On eye-pieces for telescopes and microscopes / [Cornelius Varley]. Source: Wellcome Collection.

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    shews two lateral pencils kk arriving at the eye-lens with the imperfections just stated ; this lens, besides rendering them parallel, deflects them towards the axis, so that the eye receives them at the same time with the central pencils; but the eye-lens has deflected the blue rays, as shewn by the dotted lines, more than the red, and this will cause the image, as viewed by the blue rays, to appear under a larger angle than by the red rays, hence arises the blue margin to the field, and a blue inner and red outer margin to every dark feature in the field of view, and the eye-lens can only render one of the colours in each pencil parallel, as shewn in fig. 1 ; all the others will either converge or diverge a little. From this error decreasing towards the centre till it leaves only the first- named error in the central pencil, opticians have been in the habit of saying that all eye-lenses perform well at their centre, whereas they only perform less ill as their aper¬ ture is reduced, and not perfectly well even at the centre. Single eye-lenses will bear only a small aperture on account of the lateral error increasing with the diameter, and this small aperture, taking but few pencils from the object-glass, gives but a small field. The object-glass gives the image in the curve be, fig. 1, whilst the dotted curve gh is demanded by the eye-lens, to enable us to see the whole image distinctly; so that, in addition to its having a small field, a small part only is distinct at one time. I will now shew the means by which these errors are either corrected, or very much reduced. Compound object-glasses are constructed to prevent dispersion, and thus bring all the differently coloured rays to the same point; from this property they are called achromatic. But such an object-glass, though it does in practice, will not mathematically, serve the purpose, be-