THE CORNEA, IRIS AND AQUEOUS HUMOR 815 



of depressions of the nervous centers, as well as in all conditions of ner- 

 vous exhaustion, deep narcosis and comas. In dyspnea the pupils are 

 large, but become smaller if this condition is changed into asphyxia. 

 They are also enlarged by sensory impulses from the digestive and 

 sexual organs, as well as by somatic and visceral sensations of pain. 

 Even the cerebral cortex may influence their size without any apparent 

 peripheral stimulation. Thus, it has been shown by Haab that if a 

 person is made to look at a dark wall, while his eyes are illuminated 

 by a constant light placed laterally in front of him, a marked constric- 

 tion of his pupils results whenever his attention is called to the light. 

 Quite similarly, his pupils may be made to dilate at any time by 

 drawing his attention to the dark wall. Some persons, indeed, are 

 able to constrict and to dilate 

 their pupils by merely calling up 

 a mental picture of bright and 

 dark objects. 



Spherical Aberration. In dis- 

 cussing the focal points formed 

 by spherical lenses, we have as- 

 sumed that the rays emitted by a 

 luminous object are sharply inter- 

 sected behind the lens. Strictly 

 speaking, this is not true, because 

 the refraction of a lens differs 



somewhat in its different zones for FlQ 423 . DIAGRAM ILLUSTRATING 



the reason that its prismatic con- SPHERICAL ABERRATION. 



Stituents are not centered with The iris being retracted the rays of light 



Sufficient accuracy to act in per- P ass throu f the outer zone of the lens and 



., r are more sharply refracted than those tra- 



fect unison. Ihe most perfectly versing its center. 



refracting portion of a lens is its 



central area, having an aperture not exceeding 10 to 12. If the size 

 of this aperture is increased so that the rays can also traverse its 

 peripheral segments, these rays will be brought to a focus in front of the 

 focal point of those refracted through its center (Fig. 423). The in- 

 tersections of these aberrated rays are called caustics. Obviously, 

 their presence must render the image indistinct. This condition which 

 is called spherical aberration, is also present in the lens of our eye, but 

 is prevented from interfering with the formation of the retinal image 

 by the fact that its peripheral extent is usually covered by the margin 

 of the iris. In this regard, therefore, the latter performs the func- 

 tion of a stop, i.e., it cuts off the rays from the circumference of the 

 lens and allows only the passage of a concentrated central beam. 



Chromatic Aberration. If a bundle of. light is projected through a 

 lens, it will be noted that the rays traversing its central segment appear 

 on its other side chiefly as white light, while those passing through its 

 peripheral zone are dispersed into their different colored components. 

 The image then appears surrounded by a colored margin. This effect 



