1028 



VISION. 



by altering the size of the object ; /? is then a constant for the instrument ; 

 and y being found, the radius is calculated from the same formula. In 

 Landolt's ophthalmometer the doubling of the object is brought about by- 

 means of two prisms, while in the instrument of Mandelstamm and Schbler 

 the reflected image is directly measured by means of microscope and 

 micrometer. 



Position and curvature of the anterior lens surface. — The distance 

 between the anterior surface of the cornea and the anterior surface 

 of the lens was measured by Donders with the corneal microscope. In 



Fig. 373. 



the method used by Helmholtz advantage was taken of the facts that the 

 anterior lens surface corresponds with the plane of the pupil, and that 

 the reflected image of an object in the corneal axis appears to lie behind 

 the cornea at a distance equal to half its radius of curvature. With the 

 ophthalmometer at g' in Fig. 373, the corneal reflection of a luminous 

 object at b is doubled. The pupil will be seen double at the same time, 



and a fixation point, /, is moved till a position of 



the observed eyeball is found at which the doubled 



corneal images correspond with the edges of the 



doubled pupil (Fig. 374). This is repeated with the 



ophthalmometer at g 2 , and the point where the 



lines g'a and g 2 a intersect will show the position 



of the pupil. In calculating the position, account 



has to be taken of the difference between the real 



and apparent positions of the pupil and of the angle fah, between the 



corneal axis and the line of vision (see p. 1031). The distance is found 



be about 3*6 mm., the figures given by different investigators varying 



from 29 to 40 mm. 



Fig. 374. 



