200 Dr Knox on the Limits of the Retina, <$c. 



in their progress to the retina. I have examined this mem- 

 brane with the microscope, and find it exceedingly opaque ; 

 still it may be insufficient to prevent the rays of light from 

 reaching the retina, or rather, (as we know not yet the nature 

 of light sufficiently well,) we may say, that the membrane re- 

 ceives an impression which is thus communicated to the con- 

 tiguous retina. The absolute opacity of the pigmentum, in 

 the eye of the cuttle-fish, may even be doubted, and we know, 

 by a very simple experiment,* that its immediate vicinity, or 

 rather contiguity, with the retina, may possibly permit lumi- 

 nous rays to penetrate to the retina itself. 



The observations on the mode in which vision is performed 

 in these animals are altogether distinct from the anatomical 

 details which I have been particularly careful in submitting to 

 the Society, precisely as they appeared to me at the time of 

 the dissections. The considerations which arose out of the 

 facts I have stated, and out of those already known relative to 

 the singular construction of the lens in this class of animals, 

 in so far as they modify the received theories of vision, will 

 shortly be submitted to the Society, by a gentleman whose 

 extensive knowledge of optics eminently qualify him for so 

 difficult and obscure an inquiry. 



It seems proper to add, that most of the dissections, whose 

 results are detailed in the preceding memoir, were performed 

 in the presence of several friends, and that I had the honour 

 of demonstrating the principal anatomical facts to the present 

 distinguished Secretary of the Society. I shall farther endea- 

 vour, in order as much as possible to perfect the view I have 

 taken of the anatomy of the eye of the cuttle-fish, to submit 

 to the Society, at a future meeting, accurate drawings of the 

 whole of the anatomical appearances in the order of dissec- 

 tion. 



• The experiment alluded to consists merely in holding a dark coloured 

 handkerchief or piece of black crape at a short distance before the eyes, so 

 as completely to intercept the rays of light, and, comparing the total dark- 

 ness so produced, with the tolerably distinct vision which follows, when 

 the crape, instead of being placed at a distance from the eye, is brought in 

 almost immediate contact with the cornea. 



