830 OPHTHALMOMETER. ERECT IMAGES. 



from -any point of the object A B. As all the axial rays must pass 

 through the common nodal point k, this is also called the ' 'point of in- 

 tersection of the visual rays." 



In the enucleated eye of an albino, or in any eye in which a piece of the sclera 

 and choroid has been replaced by a piece of glass, the inverted image may be readily 

 seen. In fact, the inverted image of a candle held in front of the eye may be 

 seen through the sclera, if the eye is turned strongly to one side, and if it contains 

 but little pigment. 



By means of the construction of the retinal image the size of this image may 

 be easily calculated, if the size of the object and its distance from the cornea are 

 known. As the two triangles A B k, and cdkare similar, obviously A B :c d = 

 f k : k g. Therefore, c d = (A B X k g) : f k. All of these values are known: 

 namely kg = 15.16 mm. ; further fk=ak + af, of which a f may be measured 

 directly and a k =7.44 mm. The size of A B is obtained by measurement. 



The angle A k B is designated the visual angle; the angle c k d is, of 

 course, equal to it. It may be readily seen that the objects x y and r s, 

 situated nearer the eye, must have the same visual angle. For this 

 reason all three objects A B, x y, and rs have a retinal image of the 

 same i'size. Objects whose peripheral points, when united with the 

 nodal point, subtend a visual angle of the same size, and which conse- 

 quently have retinal images of the same size are said to have the same 

 "apparent size." 



FIG. 280. Ophthalmometer (after v. Helmholtz). 



For the determination of the optical cardinal points by the method 

 of Gauss a knowledge of the following relations is necessary : 



1. The indices of refraction are : for the cornea 1.3739, f r the aqueous 

 humor and the vitreous humor 1.377, f r the lens 1.4545 (the mean 

 value of the different layers), air being taken as i, and water as 1.33 



2. The radii of the spherical refracting surfaces are: for the cornea 

 7.7 mm. ; for the anterior surface of the lens 10.3 ; for the posterior surface 

 of the lens 6.1 mm. 



3. The distances between the refracting surfaces are : from the anterior 

 surface of the cornea to the anterior surface of the lens 3.4 mm. ; from 

 the latter to the posterior surface of the lens (axis of the lens) 4 mm. ; 

 the diameter of the vitreous body is 14.6 mm. The total length of the 

 optical axis is therefore 22.0 mm. 



As it is impossible to measure the normal curvatures of the eye after death 

 curately, on account of the rapid collapse of the eye, the calculation of the 

 the refracting surfaces is made according to Kohlrausch's method, from 

 ne size 01 the reflected images in the living eye. The size of a luminous object is 

 the reflected image as the distance of each to half the radius of the 

 The size of the reflected image must, therefore, be determined, 

 rement is made by the ophthalmometer of v. Helmholtz. The ap- 

 :ted on the following principle: If an object is seen through an 

 glass plate, it appears to be displaced laterally. This displacement 



