Possibilities of Retinoscopy 19
ment. In astigmatism the movement is different in different meridians. To determine the degree of the error, or to distinguish accurately between hyperme-tropia and normality, or between the different kinds of astigmatism, it is usually necessary to place a glass before the eye of the subject. If the mirror is concave instead of plane, the movements described will be re versed; but the plane mirror is the one most commonly used.
This exceedingly useful instrument has possibilities which have not been generally realized by the medical profession. Most ophthalmologists depend upon the Snellen1 test card, supplemented by trial lenses, to deter mine whether the vision is normal or not, and to deter mine the degree of any abnormality that may exist. This is a slow, awkward and unreliable method of testing the vision, and absolutely unavailable for the study of the refraction of the lower animals, of infants, and of adult human beings under the conditions of life.
The test card and trial lenses can be used only under certain favorable conditions, but the retinoscope can be used anywhere. It is a little easier to use it in a dim light than in a bright one, but it may be used in any light, even with the strong light of the sun shining di rectly into the eye. It may also be used under many other unfavorable conditions.
It takes a considerable time, varying from minutes to hours, to measure the refraction with the Snellen test card and trial lenses. With the retinoscope, however, it can be determined in a fraction of a second. By the
1 Herman Snellen (1835-1908). Celebrated Dutch ophthalmologist, pro fessor of ophthalmology in the University of Utrecht and director of the Netherlandic Eye Hospital. The present standards of visual acuity were proposed by him, and his test types became the model for those now in use.