The shadow-test in the diagnosis and estimation of ametropia / by W.M. Beaumont.

Date:
1890
Catalogue details

Licence: Public Domain Mark

Credit: The shadow-test in the diagnosis and estimation of ametropia / by W.M. Beaumont. Source: Wellcome Collection.

Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.

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    fering with the transmission of light) and a slow moving shadow mean a high degree of error ; a bright reflex and a quick moving shadow tell us there is but little error. The higher the error then the more difficult is it to see the shadow. The shape of the shadow is said to vary with the amount of error, a linear shadow being commonly stated to be synonymous with a low degree of ametropia, whilst a crescentic one corresponds with a high degree. The student is not, however, advised to lay too much import- ance on the differentiation of the degree by means of the shape of the shadow. In some forms of irregular astigmatism, such as in conical cornea, it is almost impossible to tell either the shape of the shadow or the direction of its movements. For a full optical explanation of the cause of the shadow the reader is referred to Dr. Charnley's classical exposition in the tenth volume of the Royal Ophthalmic Hospital Reports. With regard to the mirror, either a plane or a concave one may be used, but the shadow produced by the plane mirror will be found to move in exactly the opposite direction to that produced by the concave mirror, except- ing only in very low degrees of myopia. Which mirror then shall we use f There are advantages and disad- vantages connected with each. The plane mirror was used by Cuignet, and is preferred by many ophthalmo- logists. It is simpler in practice, and there are fewer exceptions to remember in its application, but it is rather more difficult for the beginner to throw the ray of light in the right direction, and it has the great disadvantage that