Squint : its causes, pathology and treatment . But if there be any heterophoria itwill now be able to cause the eyes to deviate, therelative positions of the streak and light indicatingthe direction and degree of the deviation. produce extreme distortion in one direction only, a wellmarked vertical line can be seen through the rods whentheir axes are horizontal, and a horizontal line can be seenwhen their axes are vertical. Care should therefore betaken that there be no prominent horizontal or vertical linesnear the centre of the field of vision. For this reason, thepaper on which the tangent
Squint : its causes, pathology and treatment . But if there be any heterophoria itwill now be able to cause the eyes to deviate, therelative positions of the streak and light indicatingthe direction and degree of the deviation. produce extreme distortion in one direction only, a wellmarked vertical line can be seen through the rods whentheir axes are horizontal, and a horizontal line can be seenwhen their axes are vertical. Care should therefore betaken that there be no prominent horizontal or vertical linesnear the centre of the field of vision. For this reason, thepaper on which the tangent scale is printed should be ofnearly the same colour as the background on which it ishung: or the large figures may be marked on the wallitself. With these precautions, I have always found therod test quite reliable. 172 HETEROPHORIA The tangent scale is shown in fig. 20. Thelarge figures on the horizontal and vertical scalesdenote tangents to degrees at a distance of 5metres. A small electric or other light is placedat the zero of the Fig. 21. The Maddox double prism (fig. 21) consists oftwo prisms, each of 40,1 cemented base to this double prism is placed with its apicesvertical, before one eye, so that the line of junc-tion of the bases crosses the pupil horizontally, 1 To avoid confusion, the strength of a prism is always,in this book, denoted by the number of degrees which itdeflects a ray of light. Chromatic dispersion produces 110appreciable error in the weak prisms used in ophthalmology. An optician usually numbers a prism according to thewidtli of its geometrical angle (the angle between the twoplane surfaces). The refracting power of such a prismvaries according to the kind of glass of which it is practical purposes it may be taken as half the geo-metrical angle. For instance, a prism which deflects .1 rajof light to the extent of 4 will have a geometrical angleof about 8°. HETEROPHORIA IJ$ two false images of any small object will be seen,o
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