396 Of Mgbf, or Vifion. [Book itf. 



will be proportionably Icfs, and therefore the object will 

 he diminimed in the lame proportion. 



From fome late experiments made by Dr. Hofach 

 and Mr. Ramfden, it appears, that the power of 

 changing the focus of the eye, and adapting it- to dif- 

 ferent diftances, does not re fide in the cryftalline lens, 

 bu: in the cornea: that the cornea is competed ofla- 

 rmWi that it is elaftic, and capable of being elongated 

 one eleventh of its diameter, and of contracting to its 

 former length by its own exertions ; and laftly, that 

 the tendons of the four (trait mufcles of the eye 

 are continued to the edge of the cornea, and termi- 

 nate or are inferted in its external lamina. By the 

 fame experiments it was found, that in changing the 

 focus of the eye from feeing with parallel rays to a 

 near diftance, there is a vifible alteration produced in 

 the figure of the cornea, which renders it more convex, 

 and the alteration by which the cornea is brought 

 back to its former Hate is equally vifible*. 



Artificial eyes are fold by the opticians, in which all 

 the humours are made of different kinds of glafs, and 

 may be feparated at pleafure. At the back part, where 

 the retina is fuppofed in the natural eye to receive the 

 converged rays a is placed a piece of ground glafs, 

 where the image from the oppofed objects is painted 

 in an inverted pofition, as in a camera obfcura. The 

 iame effect may be produced with a natural eye, and 

 the nature of vifion may be thus experimentally de- 

 monftrated: if a bullock's eye is taken frefh, the pof- 

 terior coats dexteroufly removed even to th* vitreous 

 humour, and if a piece of white paper is then placed at 

 the part, the image of any bright object which is placed 

 before the eye -will be feen diftinctly painted on the 

 paper, but in an inverted pofition. 



* 'Phil. Tranf. for 1795, Part. i. 



It 



