POWEE AND 8TKUCTUKE OF THE EYE. 9 



only of other worlds belonging to the solar sys- 

 tem, but of other systems of worlds infinitely 

 removed into the immensity of space ; and when 

 aided by the telescope, the invention of human 

 wisdom, it is able to discover the forms, the 

 phenomena, and the movements of bodies whose 

 distance is as inexpressible in language as it is 

 inconceivable in thought. 



While the human eye has been admired by 

 ordinary observers for the beauty of its form, the 

 power of its movements, and the variety of its 

 expression, it has excited the wonder of philoso- 

 phers by the exquisite mechanism of its interior, 

 and its singular adaptation to the variety of pur- 

 poses which it has to serve. The eyeball is 

 nearly globular, and is about an inch in diameter. 

 It is formed externally by a tough opaque mem- 

 brane called the sclerotic coat, which forms the 

 white of the eye, with the exception of a small 

 circular portion in front called the cornea. This 

 portion is perfectly transparent, and so tough in 

 its nature as to afford a powerful resistance to 

 external injury. Immediately within the cornea, 

 and in contact with it, is the aqueous humour, a 

 clear fluid, which occupies only a small part of 

 the front of the eye. Within this humour is the 

 iris, a circular membrane, with a hole in its centre 

 called the pupil. The colour of the eye resides 

 in this membrane, which has the curious property 

 of contracting and expanding so as to diminish 

 or enlarge the pupil, an effect which human 

 ingenuity has not been able even to imitate. Be- 

 hind the iris is suspended the crystalline lens, in 

 a fine transparent capsule or bag of the sam^ 

 form with itself. It is then succeeded by the 



