RETINAL CHANGES INVOLVED IN VISION 



563 



that the distance betweeiFthe vessels and the sensitive elements of the i^tina 

 must amount to between O17 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 microscope and looking at its 

 external surface we see that the rods and cones form a sort of mosaic, the 

 thicker cones Jbeing surrounded by the smaller circles representing the 

 cross- sections^ of the rods. Singe each of these is a terminal sense-element 

 the image thrown by the dioptric 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 be formed by a synthesis of the 

 elementary sensations produced by the stimulation of every single rod or 

 cone cell. 



DIRECT AND INDIRECT VISION 



>y 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 

 i i>t ; na. 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 repre- 

 sents the extent of the visual field 

 in which we have distinct vision. 

 The light which falls into the eye 

 forms an image of external objects 

 which extends over the whole of 

 the retina. The sensations excited 

 by the stimulation of the periphery 

 of the retina are much more in- 

 distinct than those excited by the 

 image on the central spot. The 

 appreciation of external objects, 

 by means of the image, they throw 

 on the external parts of the retina, 

 is spoken of as indirect vision in 

 contradistinction to direct vision, 

 which implies fixation of the object 

 and the formation of an image of 

 it on She fovea centralis. The whole extent of the objects which we can 

 see by direct and indirect vision is spoken of as the visual field. In order 

 to determine the visual field we make use of a perimeter (Fig. 290). 



This instrument consists of a band of metal forming the arc of a circle of about 

 35 cm. radius. At one end this arc is fastened to a pillar, and can be turned through 

 the axis passing through the pillar so as to lie in various meridians. At the centre 

 of the circle is another pillar, which provides a chin-rest, so arranged that the eye 



FIG. 293. Priestley Smith's perimeter. 



