VISION. 285 



do so when light is thrown into the eyes. The nerve centers for 

 the pupil are very sensitive to general nervous conditions, thus 

 accounting for the dilatation of the pupil which occurs during 

 fright or other emotions, or pain. The pupils are contracted in 

 the early stages of asphyxia or anesthesia, as in the early stages 

 of nitrous oxide administration, but they become dilated when 

 the anesthesia or asphyxia becomes profound. Their condition 

 helps to serve as a gauge of the depth of anesthesia. 



Imperfections of Vision. The optical system of the eye is 

 not perfect. Some of these imperfections exist in every eye, 

 whilst others are only occasional. The errors in every eye are 

 those known as sperical and chromatic aberration. Spherical 



Fig. 54. A, spherical aberration. The rays which strike the margins of 

 the lens are brought to a focus before those striking near the center. B, 

 Chromatic aberration. The ray of white light (W) is dissociated by the 

 lens into the spectral colors, of which those at the red end (R) are not 

 brought to a focus so soon as those at the violet end (V). 



aberration (Fig.- 54), occurs because the edges of the lens have 

 a higher refractive power than the center, so that the image on 

 the retina is surrounded by a halo of overfocused rays. Chro- 

 matic aberration is due to the fact that white light, on passing 

 through the lens, suffers some decomposition into its constituent 

 colored rays (the rainbow colors), of which certain (viz. r those 

 towards the violet end of the spectrum) come to a focus sooner 

 than others (viz., those towards the red end), thus creating a 

 colored edge on the focused image. These errors are greatly 

 minimized, although not entirely removed, by the pupil, which 

 cuts out the peripheral rays. 



The occasional errors are long-sightedness or hypermetropia, 



