RETINAL CHANGES INVOLVED IN VISION 



627 



r, 



the layers of the retina are absent except the outermost, i.e. that of 

 the cones. 



(c) ' Purkinje's figures.' If a strong light be focused by means of 

 a lens on to the sclerotic just outside the cornea, and the eye be made 

 to stare fixedly at a dull blackground, an arborescent image of the 

 retinal vessels will appear on the background. On moving the 

 illumination the image of the vessels will move in the same direction. 

 Knowing the dimensions of the eyeball and the distance of the back- 

 ground from the eye as well as the angle through which the light is 

 moved and the apparent displacement ot the image of the vessels, 

 the distance of the sensory part of the 

 retina behind the vessels may be calculated 

 (Fig. 287). Direct measurements in this 

 way have shown that the distance between 

 the vessels and the sensitive elements of the 

 retina must amount to between 0-17 and 

 0-36 mm. Anatomical measurements of the 

 thickness of the retina show also that the 

 average distance between the vessels and 

 the layer of rods and cones varies between 

 0-2 and 0-3 mm., showing that it is in this 

 layer that the actual transformation of a 

 light stimulus into a nerve impulse must 

 take place. 



On spreading out the retina under the 



A, a shadow is formed on the 

 hinder layers of the retina at 

 a'. This is projected along a 

 line passing through the optic 

 axis, and appears to come from 



FIG. 287. Diagram of the 

 path of the rays of light in 

 the formation of Purkinje'* 

 figures. 



. v represents a retinal vessel. 



microscope and looking at its external When this is illuminated from 



surface we see that the rods and cones 



form a sort of mosaic, the thicker cones 



being surrounded by the smaller circles 



representing the cross-sections of the rods, a point (a")~oii the wall. On 



Since each of these i. a terminal ^sense- JZ*tfg!* 



element the image thrown by the dioptric to move from a" to V. 



mechanism of the eye on to the retina must 



be converted into a mosaic-like expanse of small isolated pictures, and 



our impression of external objects must in every case be formed by a 



synthesis of the elementary sensations produced by the stimulation of 



every single rod or cone cell. 



DIRECT AND INDIRECT VISION 



If we fix our attention on to an object, we direct our eyes so that 

 the image of the centre of the object falls exactly on the fovea centralis 

 of each retina. The diameter of the central spot is about 1 to 1-5 mm., 

 which corresponds to a visual angle of 4 to 6. This angle therefore 

 represents the extent of the visual field in which we have distinct vision, 



