388 LONG AND SHOET SIGHT. 



the aqueous humor, the lens, and the vitreous humor act to- 

 Chromatic ab- ^ . . 

 erration is un- gether in the same manner as the different pieces of glass in 



corrected. an acnroma tic arrangement. Optical reasons, however, found- 

 ed upon the constitution and refractive powers of those substances, lead 

 us to abandon that view, and in a theoretical respect to regard the eye 

 as imperfect in this particular. 



Adjustment for the variable intensity of light is effected by the dilata- 

 tions and contractions of the iris, the pupillary opening; of 



Adjustment for . 7 i P i -, 



the intensity which varies from the -?fc to the -J of an inch in diameter. 

 We are thus enabled to bring to the same degree of illumin- 

 ating effect upon the retina lights which differ in brilliancy in the pro- 

 portion of one to forty-five. The means by which this is accomplished 

 will be more particularly described when we speak of the nervous mech- 

 anism of the eye. 



It has been already observed that the actual field of view at a given 

 moment is quite limited. We are liable to deceive ourselves on this 

 point from the rapidity with which the eyeball can be directed to differ- 

 ent parts in succession. 



In what has been said, reference is made to a perfect eye ; but imper- 

 Long and short fections are very common. Two may be more particularly 

 cSre^donb'y 11 P ointed out long-sightedness and short-sightedness. In 

 spectacles. the former, objects, to be seen distinctly, must be placed 

 farther off than the usual distance ; in the latter they must be brought 

 nearer. Long-sightedness arises from the flatness of the lens or cornea, 

 so that the focal images given do not fall truly on the black pigment, but 

 would be, at a certain distance, exterior to it ; hence the indistinctness 

 that results. Short-sightedness is due to an excess of curvature in the 

 cornea or lens, the rays forming their focal images before the black pig- 

 ment is reached. The former defect may be removed by the use of con- 

 vex lenses as spectacles, the latter by concave. It is often said that 

 short-sightedness is a defect of early life, long-sightedness of old age. 

 However this may be in another respect, it is not so optically. Indeed, 

 cases sometimes occur in which one eye is affected with the former and 

 the other with the latter difficulty. Very frequently the two eyes, com- 

 pared together, will be found differently advanced in their degree of im- 

 perfection, and hence the difficulty of obtaining a pair of spectacles, 

 though the selection is attempted to be made out of a large assortment. 

 In such cases, each eye should be accommodated with a lens to suit it- 

 self. 



Compared with the organ of hearing, the eye is much more limited in its 

 Limit of vision action ; for, while the ear can distinguish sounds which vary 

 is one octave, through many octaves, the eye can only perceive vibrations 

 which, to use the language of acoustics, differ by a single octave only. 



