How to use the ophthalmoscope : being elementary instructions in ophthalmoscopy, arranged for the use of students / by Edgar A. Browne.

  • Browne, Edgar A. (Edgar Athelstane), 1841-1917.
Date:
1891
Catalogue details

Licence: Public Domain Mark

Credit: How to use the ophthalmoscope : being elementary instructions in ophthalmoscopy, arranged for the use of students / by Edgar A. Browne. Source: Wellcome Collection.

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    point were to be situated at F, the rays would be reflected to L. These two points are called con- jugate foci, and have a constant relationship, so that (with the principal focal length known) when the distance of one is given the other can be found by a mathematical formula. Their relative positions may be easily remembered. (a) If the luminous point is at the centre, the conjugate focus is also at the centre. (/3) If between principal focus and centre, the conjugate is beyond the centre. (7) If beyond the centre, the conjugate is be- tween the principal focus and centre. With convex mirrors all the above facts are re- versed, but require attention as introducing a new and important princijale. Hitherto we have fol- lowed the rays, and seen them converge to a real or positive focus. In the case of a convex, spherical surface, parallel rays are rendered divergent, and therefore no positive or real focus is formed. But if the divergent rays are produced by imaginary lines on the far side of the mirror, they will be found to meet in a point corresponding to the principal focus of a concave mirror of the same curvature. As the focus is imaginary, and on the opposite side of the mirror from the actual rays, it is termed negative, and as the divergent rays have the same
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