Volume 10

The American encyclopedia and dictionary of ophthalmology / edited by Casey A. Wood, assisted by a large staff of collaborators.

Date:
1913-1921
Catalogue details

Licence: In copyright

Credit: The American encyclopedia and dictionary of ophthalmology / edited by Casey A. Wood, assisted by a large staff of collaborators. Source: Wellcome Collection.

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    those that exist when they are in normal position before the eye, except that the course of the light is reversed. For a duplication of ocular conditions, there is provided an artificial eye with refractive elements of such power as to image a point of the retina on the far point sphere. This action is the same for all directions of the line of sight; therefore, for the sake of simplicity, both the retina and the refractive elements of the eye are eliminated, and only the far point sphere is considered, and represented by a concave disc provided with printed tablets. See the figure. An observing telescope is mounted on a ball-and-socket joint, the center of which represents the center of rotation of the eye. The ball The Lens Comparator. is bored out to permit the passage of the light and contains a dia- phragm which limits the diameter of the pencil of light to a value approximately equal to the aperture of the pupil of the eye. By means of this ball-and-socket joint, the telescope can be moved around the center of rotation in any direction within an angle of 60°, so that the lenses mounted for eomparison on a revolving disc can be tested in succession for any line of sight within a solid angle of 60°. These lenses are situated between the diaphragm and the telescope at a dis- tance of 25 mm. from the center of rotation. When testing the lenses in succession, the centering of each is accomplished by a spring catch, thus making possible a quick comparison between them. The course of the light pencils is understood when we assume the ophthalmic lenses to be simple spheres. The center of the concave