NATURAL HISTORY. 45 



1 'Tis greatly wise to -walk with our best hours, 

 And ask them what report they've borne to heaven." YOUNG. 



130. It is customary with opticians to place in the hands of a person requiring 

 spectacles a book or newspaper, and by the distance at which the one or the othei 

 is instantly held from the sight, the optician is enabled to judge of the degree of tho 

 defect of the vision and the kind of glass that is required. 



131. Why does the eye lose its lustre in old age and sickness ? 

 Because the lustre of the healthy eye is produced by the aetion of 



the muscles of the eye-ball maintaining it firmly in its place, pres- 

 sing it against the fatty cushion on which it rests, and giving to it 

 that tension necessary to render the corner prominent, clear, and 

 full. In old age and sickness the muscles of the eye partake of the 

 general debility of the system, and its functions are thus deranged. 



132. Wliy do some persons squint ? 



Strabismus, or squinting, is caused by one or more of the muscles 

 of the eye being shortened or elongated ; and by the derangement 

 of their action the pupil is consequently carried out of the proper 

 axis of vision. 



133. This defect is cured by a very simple operation. The shortened or elongated 

 muscle is cut down by a skilful operator. The false position of the pupil is imme- 

 diately rectified by the division of the disordered muscle that induced obliquity by its 

 irregular action. The divided muscle in healing shortens or lengthens, and during 

 the healing process suits itself to the exigencies of the case, and in a short time 

 re-unites. 



134. Wliy when we look for some time at a white spot upon a 

 black ground, and afterwards direct the eye to a white ground, do we 

 perceive a black spot ? 



Because the retina of the eye has become insensible in the point 

 which was formerly fatigued by the white light. In the same 

 manner, after the retina has been some time without acting in one 

 of its points, whilst the others have icted, the point which has been 

 in repose becomes of an extreme sensibility, and on this account 

 objects seem as if they were spotted. 



135. In this manner it is explained why, after having looked for a long time at 

 a red spot, white bodies appear as if spotted with green ; in this case the retina has 

 become insensible to the red rays, and we know that a ray of white light, from 

 which the red is substracted, produces the sensation of green. 



