428 ORGANS OF THE SENSES. 



its greatest length, while the ocular cone will become as 

 short as it is possible for it to be. If, on the other hand, 

 the luminous rays proceed from an object very near the 

 eye, the objective cone will be very short, while the ocular 

 cone which it produces in the eye will be much longer 

 than in the former case. We see, therefore, that it is only 

 when at a certain distance from the luminous object that the 

 ocular cone is of such a length that its summit falls exactly 

 on the retina ; in all other cases, the luminous point being 

 farther from, or nearer to, the eye, the ocular cone produced 

 will be either too long or too short, and its summit will con- 

 sequently be either before or behind the retina : in short, the 

 luminous point will be represented on the retina, not as a 

 point, but as a small circle, called the circle of diffusion^ and 

 the image resulting from it will be confused. 



What would happen in a physical organ such as we have 

 described does not, however, take place in the eye, in its 

 normal state. Whatever (within certain limits) be the dis- 

 tance of the luminous point, we have the power of bringing 

 the summit of the ocular cone produced by the rays of light 

 proceeding from it, directly upon the retina: a man can 

 alternately look at, and see with nearly equal distinctness, a 

 star and the end of his nose. In short, we have the power of 

 adapting or accommodating our eye to different distances. 



C. Adaptation or Accommodation. The method of adap- 

 tation or the exact coincidence of the summit of the ocular cone 

 with the retina, has been only recently and exactly defined. For 

 a long time the existence of this accommodation was denied. 

 It may, however, be proved in several ways. If, for instance, 

 we hold up two fingers, one behind the other, at a certain 

 distance, and fix our attention on one, we shall find that we 

 see distinctly only this one, the eye being adapted to see only 

 one and not the other, which appears vaguely defined ; this 

 is because, at this moment, one of the two fingers is distinctly 

 painted on the retina, while the other only produces circles 

 of diffusion upon it. This fact is still more clearly demon- 

 strated by a celebrated experiment made by Scheiner, and 

 which consists in placing before the eye a card in which two 

 small holes are pierced near each other (Mm Nn, Fig. 115), 

 and then looking through them at two luminous points (the 

 heads of two pins, for instance) placed one before* the other, 

 at a certain distance (as with the two fingers in the fore- 

 going experiment) : if we look at one of these points fixedly, 

 we shall see the latter double. The reason of this is owing to 



