214 



SENSE OF SIGHT. 



Fjg - 83 - 



ABC 



DEF 



Aberration of Refrangibility. 



Newton was of opinion, 

 that the light, in travers- 

 ing a refracting medium, 

 always experiences a dis- 

 persion of its rays, pro- 

 portional to its refraction. 

 He therefore believed, 

 that it would be impos- 

 sible to fabricate an 

 achromatic glass. This 

 is one of the rare cases 

 in which that illustrious 

 philosopher erred. Since 

 his time and chiefly by 

 the labours of Mr. Dollond instruments have been formed on the prin- 

 ciples above mentioned, so as to greatly diminish the inconveniences 

 sustained from the use of common lenses; although they may still not 

 be perfectly achromatic. The inconvenience is farther obviated by the 

 diaphragm in telescopes, already referred to. As the dispersion is most 

 experienced near the margin of the lens, it shuts off the rays, which 

 would otherwise fall upon that portion, and diminishes the extent of 

 aberration. The human eye is achromatic. It is obviously essential 

 that it should be so; and this result is owing to a combination of causes. 

 It is formed of media of different dispersive powers. Its lens is con- 

 stituted of layers of different densities, and it is provided with a dia- 

 phragm of singularly valuable construction. 



Such are the prominent points of the beautiful science of optics, that 

 chiefly concern the physiologist as an introduction to vision. Others 

 will have to be adverted to, as we consider the eye in action. 



2. ANATOMY OF THE ORGAN OF VISION. 



The human eye is almost spherical, except for the prominence at its 

 anterior and transparent part the cornea. It has been compared to 

 a telescope, and with much propriety ; as many of the parts of that 



instrument have been added to exe- 

 cute special offices, which are admi- 

 rably performed by the eye the most 

 perfect of all optical instruments. 

 Every telescope consists, in part, 

 of a tube, which always comprises 

 pieces, capable of readily entering 

 into each other. Within this cylinder 

 are glasses or lenses, placed in suc- 

 cession from one extremity to the 

 other. These are intended to re- 

 Front View of the Left Eye-moderately fract the rays of light, and to bring 

 opened. them to determinate foci. Within 



i. Superciiia. s. cilia of each eyelid 3. the telescope is a kind of partition 



Inferior palpebra. 4. Internal canthus. 5. Kx- , . , 



ternal canthus. 6. Caruncula lachrymalis. 7. 01 paper Or metal, having a round 

 Plica semilunaris. 8. Eyeball. 9. Pupil. ftnd usually placed 



Fig. 84. 



