On throat deafness and the pathological connexions of the throat, nose, and ear / by James Yearsley.

  • James Yearsley
Date:
1853
Catalogue details

Licence: Public Domain Mark

Credit: On throat deafness and the pathological connexions of the throat, nose, and ear / by James Yearsley. 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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    line), which is exactly the combination found by Herschel to prevent spherical aberration in ordinary glass lenses. 77. There is, moreover, a provision in the optical construction of the eye to guard against chromatic aberration, by which term is understood that ten- dency of a lens to throw an indistinct image on the screen by reason of the different refrangibility of the different coloured rays of light. Thus the violet ray is most refracted in passing obliquely through a re- fracting medium, the red ray least. Supposing the screen is adjusted for focusing accurately the inter- mediate yellow or green ray, then the rays beyond that towards the violet side of the spectrum would be focused short of the screen, while the rays on the other side towards the red side of the spectrum would be focused behind the screen. In optica] instruments this chromatic aberration is got rid of by using a compound lens, formed of a convex and a concave lens of two differently refracting media ; usually a com- bination of flint and crown glass, which have different indices of refraction. A similar provision is made in the eye,* where not only is there a combination of * At least in accommodation for near vision, where the anterior surface of the crystalline lens, supposing the ordinary view of its in- creased convexity in this act be accepted, becomes more convex than