230 PRACTICAL PHYSIOLOGY 



But dK= distance of object from cornea + distance of nodal point 

 behind cornea, which latter is equal to 7*44 mm. 

 Kr is equal to 15*17 mm. 



, size of external object x 15'17 



~~ distance of object from cornea + 7 '44 



If the image be near so as to provoke a considerable effort of accommodation, 

 this equation will not represent the size of the formed image. In this case the 

 anterior surface of the lens will be more curved than in viewing more distant 

 objects, and consequently the constants for the "simple reduced eye" will not 

 hold good. The ' ' reduced eye " of Listing corresponds, strictly speaking, to the 

 lens accommodated for distant objects. 



The Ophthalmometer. This is an instrument by means of which the 

 radius of curvature of the different surfaces of the eye may be 

 measured. The degree of curvature of a reflecting surface will affect the 

 size of the image formed from some external object. If some device 

 be applied for the measurement of the image and the distance of the 

 external object from the reflecting surface be known, then the curvature 

 of the reflecting surface can be calculated. 



In Helmholtz's original form of the opthalmometer the measurement 

 of the image was achieved by causing the rays reflected from the cornea 

 to undergo deviation from their direct course by passing through glass 

 plates of a definite thickness. By introducing two glass plates, revolving 

 in a common vertical axis, two images could be obtained, and the 

 degree of overlapping of these images could be adjusted by altering 

 the angle which the two plates made with one another. The distance 

 between corresponding points in the two images could be expressed in 

 terms of the angle representing the degree of tilt of the plates and the 

 refractive index of the glass. The greater the obliquity of the plates 

 the more considerable would be the displacement of the images. 



Having obtained a value for the size of the reflected image the 

 curvature of the cornea could be calculated from the equation, 



the size of a luminous body (L) _ distance of body from cornea (d) t 

 size of its reflected image (1) \ radius of cornea (Jr) 



2dxl 



or r = f . 



LI 



A modification of Helmholtz's ophthalmometer was introduced by 

 Javal & Schiotz, in which the double glass plate was replaced by a calc- 

 spar crystal and a similar double image obtained. This was still 

 further improved by Kagenaar, who substituted compound prisms for 

 the crystal, and the instrument so cheapened and improved is generally 

 spoken of as an astigmometer. This instrument, which is essentially an 

 ophthalmometer, is represented in Fig. 210. 



