8 COLOUR VISION 



causing irregular astigmatism. For these and other reasons the retinal 

 image of a luminous point is never accurately punctate^. 



Further, the optical system of the eye is not achromatic. This fact 

 might be regarded as of extreme importance in the consideration of 

 colour vision, but in general the effect is negligible. One would also 

 expect diffraction at the edge of the pupil, and it can be demonstrated 

 to occur, especially when the pupil is small ; but this effect is also 

 negligible under most conditions. There are, however, occasions when 

 both these factors must be taken into account. 



The crystalline lens normally possesses a slightly amber-yellow hue, 

 which is inappreciable in youth, but increases as age advances. In 

 elderly people the colour of the lens causes an appreciable absorption 

 of the more refrangible rays (green, blue and violet). This fact must 

 be borne in mind in estimating the visual sensations of such people. 



The retina is a transparent membrane lining the back of the vitreous 

 chamber. It is composed of several layers, the outer of which is a 

 mosaic of rods and cones. The rods and cones are a neuro-epithelium, 

 connected with bipolar cells which form the outer nuclear layer. A 

 second set of cells forms the inner nuclear layer. The arborisations of 

 these two sets of cells form an intermediate outer reticular layer. Arbori- 

 sations from the inner nuclear layer and processes from the ganglion 

 cell layer form the inner reticular layer. The ganglion cell layer is com- 

 posed of larger cells which give rise to the axons of the optic nerve. 

 These axons form a layer upon the inner surface of the ganglion cell 

 layer, the nerve fibre layer. 



The optic disc or head of the optic nerve is situated about 3 mm. 

 to the inner or nasal side of the posterior pole of the eye. Light falling 

 on this area causes no sensory impression (Mariotte's blind spot). 



Outside the retina proper is the layer of retinal pigment epithelium. 

 It consists of cells which are hexagonal in section and have processes 

 passing forwards between the rods and cones. The cells contain minute 

 needle-shaped crystals of pigment. This layer bears some resemblance 

 to the " backing " of a photographic plate. 



External to the retina is the choroid, a highly vascular membrane 

 having the function of nourishing the outer layers of the retina. The 

 inner layers of the retina are nourished by the retinal blood vessels, 

 which spring from the disc and radiate over the surface of the nerve 

 fibre layer. The walls of the retinal blood vessels are transparent, but 



^ Parsons, " The Perce^jtion of a Luminous Point," Roy. Lond. Ophth. Hosj). Reports, 

 xvm. 239, 1012 ; xix. 104, 264, 274, 1913-4. 



