CH. LV.] VISUAL JUDGMENTS. 7^7 



tion of this subject in connection with cerebral localisation, the 

 phenomena 'of hemianopsia (p. 678), and the conjugate deviation 

 of head and eyes (pp. 679, 682). 



Fig. 598, though diagrammatic, will assist the reader in more 

 fully comprehending the paths of visual impulses, and the central 

 connections of the nerves and nerve-centres concemed in the 

 process. The fibres to the lateral geniculate body end there by 

 arborising around its cells, and a fresh relay of fibres from these 

 cells passes to the cortex of the occipital lobe. Those to the ante- 

 rior corpus quadrigeminum are continued on by a fresh relay to 

 the nucleus of the third nerve, the cells of which are also sur- 

 rounded by arborisations of the axis cylinder processes of the 

 cortical cells. 



Visual Judgments. 



The psychical or mental processes which constitute the visual 

 sensation proper have been studied to a far greater degree than 

 is possible in connection with other forms of sensation. 



We have already seen that in spite of the reversion of the 

 image in the retina, the mind sees objects in their proper posi- 

 tion, the sense of sight being here educated in great measure by 

 that of touch. 



We are also not conscious of the blind spot. This is partly 

 due to the fact that those images which fall pn the blind spot of 

 one eye are not focussed there in the other eye. But even when 

 one looks at objects with one eye, there is no blank, the area 

 corresponding to the blind spot being closed up by a mental process. 



Our estimate of the size of various objects is based partly 

 on the visual angle under which they are seen, but much 

 more on the estimate we form of their distance. Thus a 

 lofty mountain many miles off may be seen under the same 

 visual angle as a small hill near at hand, but we infer that 

 the former is much the larger object because we know it is much 

 further off than the hill. Our estimate of distance is often erro- 

 neous, and consequently the estimate of size also. Thus persons 

 seen walking on the top of a small hill against a clear twilight 

 sky appear unusually large, because we over-estimate their dis- 

 tance, and for similar reasons most objects in a fog appear 

 immensely magnified. The same mental process gives rise to the 

 idea of depth in the field of vision ; this idea is fixed in our 

 mind principally by the circumstance that, as we ourselves move 

 forwards, different images in succession become depicted on our 

 retina, so that we seem to pass between these images, which to the 

 mind is the same thing as passing between the objects themselves. 



3 K 2 



