748 THE OPHTHALMOMETER. 



passes without refraction through the nodal point, *. Further, as the focal point 

 for the luminous point, A, is upon the retina, all the rays proceeding from A must 

 reunite in d. The same is true of the rays proceeding from B, and, of course, for 

 rays sent out from an intermediate point of the body, A B. The retinal image is, 

 as it were; a mosaic, composed of innumerable foci of the object. As all the rays 

 of direction must pass through the common nodal point, k, this is also called the 

 " point of intersection of the visual rays." 



The inverted image on the retina is easily seen in the excised eye of an albino rabbit by hold- 

 ing up any object in front of the cornea and observing the inverted image through the trans- 

 parent coats of the eyeball. . 



The size of the retinal image may also be calculated, provided we know the size ot the 

 object and its distance from the cornea. As the two triangles, A B Jc and c d k are similar, 

 A B : c d = fk :kg,so that c rf = (A B, k g) : fk. All these values are known, viz., k 0-15 '16 

 mm.: farther, /Jt-a k x ,/, where a /is measured directly, and a k =7 '44 mm. The size 

 of A B is measured directly. 



The angle, A k B, is called the visual angle, and of course it is equal to the 

 angle c k d. It is evident that the nearer objects, x y, and r s, must have the 

 same visual angle. Hence, all the three objects, A B, x y, and r s, give a retinal 

 image of the same size. Such objects, whose ends when united with the nodal 

 point form a visual angle of the same size, and consequently form retinal images 

 of the same size, have the same " apparent size." 



In order to determine the optical cardinal points by calculation, after the 

 method of Gauss, we must know the following factors : 



1. The refractive indices: for the cornea, 1*377; aqueous humour, 1-377 ; lens, 

 1-454 (as the mean value of all the layers) ; vitreous humour, 1*336; air being 

 taken as 1, and water 1-335. 



2. The radii of the spherical refractive surfaces: of the cornea, 7-7 mm.; of 

 the anterior surface of the lens, 10*3 ; of the posterior, 61 mm. 



3. The distance of the refractive surfaces : from the vertex of the cornea to 

 the anterior surface of the lens, 34 mm. ; from the latter to the posterior surface 

 of the lens (axis of the lens), 4 mm. ; diameter of the vitreous humour, 14*6 mm. 

 The total length of the optic axis is 22'0 mm. 



[Kuhne's Artificial Eye. The formation of an inverted image, and the other points in the 

 dioptrics of the eye can be studied most effectively on Kuhne's artificial eye, the course of the 

 rays of light being visible in water tinged with eosine.] 



Ophthalmometer. This is an instrument to enable us to measure the radii of the refractive 

 media of the eye. As the normal curvature cannot be accurately measured on the dead eye, 

 owing to the rapid collapse of the ocular tunics, we have recourse to the process of Kohlrausch, 

 for calculating the radii of the refractive surfaces from the size of the reflected images in the 

 living eye. Tlie size of a luminous body is to the size of its reflected image, as the distance of both 

 to half the radius of the convex mirror. Hence, it is necessary to measure the size of the re- 



G 



Fig. 532. 

 Scheme of the ophthalmometer of Helmholtz. 

 fleeted image. This is done by means of the ophthalmometer of Helmholtz (fig. 532). 

 The apparatus is constructed on the following principle : If we observe an object 

 through a glass plate placed obliquely, the object appears to be displaced laterally ; the 

 displacement becomes greater, the more obliquely the plate is moved. Suppose the observer, 

 A, to look through the telescope, F, which has the plate, G, placed obliquely in front of the 

 ujfper half of its objective, he sees the corneal reflected image, a b, of the eye, B, and the 



