DIOPTRIC MECHANISMS OF THE EYEBALL 541 



where R is the radius of curvature, a ths distanca of the object, b the size of tin- image- 

 C the size of the object. The object gensrally used is the distance between two lights 

 or two white objects called mires ; the ' image ' being the distance between their images. 

 Owing to the movements of the eye the latter cannot be accurately measured by the 

 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. 269). 

 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 



n' B 



FIG. 269. Diagram to illustrate principle of 

 ophthalmometer. (After SCHENCK.) 



N 

 FIG. 270. 



obliquely, as at g l g lt there will be an apparent shifting of the line sideway to cd. In 

 the ophthalmometer there are two glass discs, g t g lt and g 2 g 2 , 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, 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 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 

 baing placed at a constant distance from the instrument and the luminous object 

 always being the same size. 



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



" Let aa (Fig. 270) be one of the plates, AB the incident, CD the refracted lay. 

 Then, since the refracted ray is parallel to the incident ray, the angle ABN is equal 



* Parson's " Elementary Ophthalmic Optics." 



