278 OUTPOSTS OF THE INTELLIGENCE SERVICE 



hyaloid uienibraiie. At the back of tlie eye, this ineinbraiie i.s in intimate 

 contact witli the retina, but on reaching the ciliary region it splits into two 

 layers, the posterior of which is continued over the anterior surface of the 

 vitreous body, and the anterior, gaining attachment to the ciliary processes, 

 becomes thickened to form the suspensory ligament of the lens. 



On the anterior surface of the vitreous body is a cavity, the hyaloid fossa, 

 in which the posterior surface of the lens is lodged. From this fossa a 

 minute canal, the hyaloid canal, passes obliquely backwards to the point of 

 exit of the optic nerve. 



Eecent work on the anatomy of the living eye with the slit-lamp seems to 

 suggest that a small space, the retro-lental space, exists between the posterior 

 surface of the lens and the anterior surface of the vitreous body, and that the 

 aqueous humour is able to j^ass between the fibres of the suspensory ligament 

 into this space, which is drained by the hyaloid canal into the lymphatics of 

 the sheath of the optic nerve, or into the retinal vessels. 



The Eye as an Optical Instrument. 



The physics of vision may be considered under two heads, 

 viz. (1) the way in which the image is produced on the retina, 

 and (2) how that image stimulates the end-organs for vision in the 

 retina so that impulses pass to the optic nerve, producing finally 

 a change in consciousness. Under the former head will fall the 

 study of the defects common to a camera and the means by which 

 they are overcome in the eye. 



(1) There can be no doubt as to the actual formation of an 

 inverted image on the retina. If a small window be cut through 

 the back of a freshly excised eye, and the space covered with a 

 sheet of tissue paper or a small bit of ground glass inserted, 

 inverted illuminated images of objects placed before the eye may 

 be seen on the screen. The optical system consists of {a) cornea, 

 [b) lens, and (c) iris. Light will undergo refraction at three 

 surfaces, e.g. where it enters the cornea and where it enters and 

 leaves the lens. This may be deduced from consideration of the 

 refractive indices of the various media of the system given below. 



Refraction also occurs at the posterior surface of the cornea, 

 but as the R.I. of cornea and aqueous humour differ only slightly, 

 we may neglect refraction at this surface. 



TABLE XXXVII 



Kefractive Indices of Media 



Air . . . . 

 Cornea 



Aqueous humour 



Lens (periphery) 



(central nucleus) 

 (total equivalent) 



Vitreous humour 



1-00 

 L37 

 L33 

 L37 

 L41 

 1-42 

 1-33 



