280 OUTPOSTS OF THE INTELLIGENCE SERVICE 



TABLE XXXIX 



mm. 

 Anterior surface of cornea to aqueous humour . . .1-15 



Anterior surface of cornea to lens ..... 3-54 

 Anterior surface of cornea to vitreous humour . . . 7-60 



Anterior surface of cornea to retina ..... 22-6 



The cornea will, therefore, have a focal length of 32 mm,, and 

 the lens of 56-3 mm. The dioptric value is, as we have seen, the 

 reciprocal of the focal length, i.e. cornea 31 and lens 18 dioptres. 

 It will be seen from these figures that the cornea plays the major 

 part in the formation of the image. When one attempts to see 

 while immersed in water, one finds it impossible clearly to perceive 

 objects near at hand, while more distant objects appear reasonably 

 distinct. Water having a refractive index of a value close to that 

 of the cornea, lengthens the focal distance of the cornea, i.e. the 

 eye becomes long-sighted and cannot bring near objects into focus. 

 When the lens is removed for cataract, the cornea has to be 

 strengthened optically by a spectacle lens of about 10 dioptres. 



The lens is an interesting structure. It is not homogeneous, 

 but is formed of a series of concentric layers of material graded in 

 optical properties, so that the refractive index increases layer by 

 layer from capsule to nucleus. The curvature of these layers also 

 increases in the same direction, i.e. the nucleus has the greatest 

 curvature, appearing almost spherical. This peculiar structure 

 gives the lens increased power. If its composition were uniform, 

 with a mean refractive index of 1-39, its power would be proportional 

 to the difference between its R.I. and the R.I, of the adjacent 

 medium, i.e. 1-39 — 1-34 = 0-05. But its actual R.I. is 1-42. 

 It has thus increased in power in the ratio 8/5. 



Focussing. 



Every one knows that in a photographic camera it is necessary 

 to adjust the distance between plate and lens in order to focus 

 sharply objects at varying distances. The eye, regarded as an 

 optical instrument, must suffer from this disadvantage, and it 

 is a matter of daily experience with us that near and far objects 

 cannot be seen clearly at the same time. How does the eye over- 

 come this difficulty ? The eyeball is rigid and the lens practically 

 fixed. No change in the relative positions of the latter and the 

 retina is possible. The adjustment, called accommodation, is 

 brought about by changes in the lens, so that the eyeball has 

 virtually a series of lenses of varying strength, from which it 

 selects the one most suited to the requirements of the moment. 



