104 SCIENTIFIC RECREATIONS 



contour of the second pin, we at last succeed, and the first once more 

 appears ill-defined. It is only since the experiments of M. Cramer and 

 M. Helmholtz that the explanation of this phenomenon could have been 



given. M. Cramer has succeeded in 

 determining on the living eye the 

 curved ray of the cornea, and of the 

 two surfaces of the crystalline lens. 

 In so doing he followed Samson's 

 method, and observed the images 

 thrown by a luminous object, whose 

 rays strike the different refracting 

 surfaces of the eye. A candle, L 

 (fig. 98), is placed before the eye, O, 



Fig. 98. M. Cramer's experiment. v y '* r , J ' 



and throws as in a convex mirror a 



straight image of the flame, A (fig. 99). The other portion of the light, 

 which has penetrated the pupil, falls on the crystalline lens, and produces 

 likewise a second straight image, B. Then the light refracted by the lens 

 reaches the posterior surface ; a portion is reflected on a concave mirror, 

 and gives the inverted image, C, very small and brilliant. M. Cramer 



observed it through a microscope, and studied the variations in 



the size of images when the eye passed from the observation of 



adjacent to distant objects. He stated : 



I . That the image, A, formed on the surface of the cornea, 



remams the same size in both cases; the form of the cornea 



therefore remains unaltered. 



2. That the image, B, formed on the upper surface of the lens, diminishes 

 in proportion as the eye is nearer the object ; the surface therefore becoming 

 more and more convex, as the focal distance diminishes a result indicated 

 by the theory that it is possible in the vision of near objects to receive the 

 image on the retina. 



3. That the third image, C, produced on the posterior surface of the 

 lens, remains nearly invariable. 



We may confirm Cramer's statements by an easy experiment. We 

 place ourselves in front of the eye of someone who looks in turn at two 

 objects placed on the same black line at unequal distances from him, and 

 are able to distinguish by the dimension of the images of the candle, which 

 object it is that he is regarding. M. Helmholtz has carried M. Cramer's 

 methods to perfection, and has been able to formulate a complete theory of 

 all the phenomena of accommodation. The laws of optics show that the 

 rays emitted by a luminous point may unite at another point by the action 

 of the refracting surfaces of the eye. Nevertheless, a white light being 

 composed of rays of diverse refrangibility, particular effects, known under 

 the name of chromatic aberration, are produced through the decomposition 

 of light, which we will proceed to study, under M. Helmholtz's auspices*. 

 * Traite tfoptique Physiologique. French translation by MM. Javal and Klein. 



