752 OF THE ORGANS OF THE SENSES, AND THEIR FUNCTIONS. 



conditions concur, and parallel rays striking the cornea are brought to a 

 focus in front of the retina, and after decussating form a circle of dispersion 

 on this membrane. All distant objects, therefore, are seen obscurely. On 



FIG. 263. 



Emmetropic Eye. Parallel rays focussed on the retina. 



Emmetropic Eye. The dotted lines showing how accommodation for the diverging rays of near 



objects is effected. 



the other hand, the patient can see objects close to his eye, because the rays 

 are there highly divergent. This condition may be remedied by placing a 

 concave lens in front of the eye, which causes the rays of light to become 

 divergent, and they are not, therefore, brought to a focus so soon in the in- 



FIG. 264. 

 C 



Hyperruetropic Eye. 



Myopic Eye. 



terior of the eye. An interesting experiment, capable of a practical appli- 

 cation in determining whether a given eye is normal or ametropic, and 

 whether the ametropia is myopic or hypcrmctropic, is that known as Sohei- 

 ner's experiment (Fig. 265); in this a card or thin piece of metal, perforated 

 with two small holes, the distance between which must be less than the 

 diameter of the pupil, is placed before the eye. If the eyes be normal, two 

 circles of light will be seen, which overlap one another to a certain extent, 

 and in this brighter part all objects appear single, because the rays are 

 united upon the retina at c. If, however, the axis of the eye be too long, as 

 in Myopia, and the retina be atp q, the rays will have crossed, and double 



