MECHANISM OF THE EYE. 149 



the retina, which is, indeed, an extension of the optic 

 nerve. These coats enclose three humours, the aqueous, 

 the vitreous, and the crystalline humours. 



The eye, in its more superficial mechanical arrange- 

 ments, presents exactly the same character as a camera 

 obscura,. the cornea and crystalline lens receiving the 

 images of objects refracting and inverting them ; but 

 how infinitely more beautiful are all the arrangements of 

 the organ of vision than the dark chamber of Baptista 

 Porta !* The humours of the eye are for the purpose 

 of correcting the aberrations of light, which are so 

 evident in ordinary lenses, and for giving to the whole 

 an achromatic character. Both spherical and chromatic 

 aberration are corrected, the latter not entirely, "and by 

 the agency of the cornea and the crystalline lens perfect 

 images are depicted on the retina, in a similar way to 

 those very charming pictures which present themselves 

 in the table of the camera obscura. 



The seat of vision has been generally supposed to be 

 the retina ; but Mariotte has shown that the base of the 

 optic nerve, which is immediately connected with the 

 retina, is incapable of conveying an impression to the 

 brain. The choroid coat, which lies immediately behind 

 the retina, is regarded by Mariotte and Bernoulli as the 

 more probable seat of vision. The retina, being trans- 

 parent, offers no obstruction to the passage of the light 

 onward to the black surface of the choroid coat, from 

 which the vibrations are, in all probability, communicated 

 to the retina and conveyed to the brain. Howbeit, upon 

 one or the other of these delicate coats a distinct image 



* The invention of the camera obscura certainly belongs to 

 Giambattista Porta, and is described in bis Magits Naturalis, sive 

 de Miraculis Rerum Naturalium, Libri Viginti; Antwerp, 1561. 

 An English translation made in 1658 exists, but I have not seen 

 it. 



Hooke, in one of the earliest volumes of the Philosophical Trans- 

 actions, describes as new many of the phenomena mentioned by 

 Porta, and particularly the images of the dark chamber. 



