602 



PHYSIOLOGY 



cannot be accurately measured by tbe usual method, employed by physicists' 

 of looking at the images through a telescope which has a micrometer at the 

 focus of the object. This difficulty is overcome by doubling the image. For 

 this purpose Helmholtz devised the ophthalmometer, in which the doubling 

 is brought about by two plane glass plates set at a variable angle to one another. 

 The principle of the instrument can be gathered from the diagram (Fig. 267). We 

 may suppose it is necessary to measure the line ab, which may be taken to repre- 

 sent an image reflected from the anterior surface of the cornea or lens. If we look 

 at this line through a plate of glass the plane of which is at right angles to our 

 line of sight, no distortion of the line ab takes place. If however the plate 

 be placed obliquely, as at g l g^, there will be an apparent shifting of the line 

 sideways to cd. In the ophthalmometer there are two glass discs, g v g l} and 

 9z 9z> one immediately over the other, so placed that the image ab is looked 

 at through the junction between the two plates. The plates are then turned, 



N 



n' B 



N 

 FIG. 268. 



as in the diagram, until ab appears as two distinct lines ec and cd just touching 

 one another at c. At this point each image of the line ab has been shifted 

 through one half the length of ab. Knowing the thickness of the plates and 

 their refractive index, it is easy to calculate, from the angle through which 

 the plates have been turned, the apparent shifting of the line ab. This lateral 



movement amounts to ac, i.e. to -- , and we have merely to double this result 



a 



in order to obtain the actual size of the image on the cornea or lens. 



A table is generally supplied with the instrument giving the actual size 

 of the image corresponding to the angle through which the plates have been 

 turned ; the eye always being placed at a constant distance from the instru- 

 ment and the luminous object always being the same size. 

 The size of the image is calculated in the following way* : 

 " Let aa (Fig. 268) be one of the plates, AB the incident, CD the refracted 

 ray. Then, since the refracted ray is parallel to the incident ray, the angle 

 ABN is equal to the angle DCN' (= a). Similarly the angle of refraction CBn 

 is equal to the angle BCri (=/3). Let h be the thickness of the glass plate. 

 Produce DC backwards to A'. It is. required to find the perpendicular distance 

 between -A and A' (= x). 



* Parson's " Elementary Ophthalmic Optics." 



