HELMHOLTZ IN KONIGSBERG 



tions from three little mirrors placed on a rod in the 

 same plane as that of the eye under examination. These 

 little specks of light are thus seen in a straight line, 

 reflected on the anterior surface, say, of the cornea, 

 and the distance between the two most distant specks 

 is the breadth of the image ; the third little speck 

 is exactly midway between the other two. The 

 ophthalmometer is then, from a suitable distance, 

 directed towards the eye, and the plates are rotated 

 until the object divides into two, and the displace- 

 ment is continued until there has been an apparent 

 movement through the breadth of the object. The 

 angular displacement is then noted, and, as already 

 stated, by the use of a formula, the size of the re- 

 flected image may be calculated in, say, millimetres, or 

 fractions of a millimetre. Finally, if the size of the 

 real object (the distance between the two mirrors 

 farthest apart on the rod), the distance of the plate 

 from the eye (the vertex of the cornea), and the size 

 of the reflected image (as measured by the ophthalmo- 

 meter), are given, it is easy to calculate the radius 

 of curvature of the reflecting surface. Helmholtz, 

 by means of this beautiful arrangement, was able to 

 show ( i ) that the radius of curvature of the cornea for 

 near and distant objects does not change ; (2) that the 

 length of the radius of curvature of the anterior sur- 

 face of the lens, when the eye looks at an object far 

 away, is 10 millimetres, and is only 6 millimetres 

 when the eye looks at a near object, that is to say, 

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