The ophthalmoscope : a manual for students / by Gustavus Hartridge.

  • Gustavus Hartridge
Date:
1907
Catalogue details

Licence: In copyright

Credit: The ophthalmoscope : a manual for students / by Gustavus Hartridge. Source: Wellcome Collection.

    19/212 (page 1)
    CHAPTER I OPTICAL PRINCIPLES INVOLVED IN THE USE OP THE OPHTHALMOSCOPE. Light is propagated from a luminous point in every plane and in all directions in straight lines; these lines of direction are called rays. Rays travel with the same rapidity so long as they remain in the same medium. The denser the medium the less rapidly does the ray of light pass through it. Rays of light diverge, and the amount of diver- gence is proportionate to the distance of the point from which they come; the nearer the source of the rays the more they diverge. When rays proceed from a distant point such as the sun, it is impossible to show that they are not parallel, and in dealing with rays which enter the eye, it will be sufficiently accurate to assume them to be parallel when they proceed from a point at a greater distance than 6 metres.