CH. LVIII.] VISUAL SENSATIONS 841 



the instrument. The blind spot is shown, and the dotted line 

 represents the normal average field of vision for the right eye. It 

 will be seen that the field of vision is most extensive on the outer 

 side ; it is less on the inner side because of the presence of the nose. 



By the use of the same instrument, it is found that the colour 

 of a coloured object is not distinguishable at the margin, but only 

 towards the centre of the field of vision, but there are differences 

 for different colours; thus a blue or yellow object is seen to be 

 blue or yellow over a wider field than a red or green object. 



In disease of the optic nerve, contraction of the field of vision 

 for white and coloured objects is found. This often occurs before 

 any change in the optic nerve is discoverable by the ophthalmoscope. 



The yellow spot of one's own eye can be rendered evident by 

 what is called Clerk-Maxwell's experiment : On looking through a 

 solution of chrome-alum in a bottle with parallel sides, an oval 

 purplish spot is seen in the green colour of the alum. This is due 

 to the pigment of the yellow spot. 



Visual Sensations. 



Visual sensations are of two kinds, colour sensations and colour- 

 less sensations. Colour sensations differ (1) in hue, for instance, blue, 

 red, yellow ; (2) in saturation, for instance, pale green and full green ; 

 this depends upon the degree of admixture with white light ; and (3) 

 in intensity, for instance, a weak sensation or a strong sensation. 

 These differences are in part dependent respectively on the length, 

 the purity, and the amplitude of the light- wave ; but they are also 

 dependent on the local or general condition of the cerebro-retinal 

 apparatus at the time of stimulation. Colours also differ (4) in 

 brightness or luminosity; this is a purely psychological quality 

 devoid of any known physical counterpart. The brightness of a 

 colour may be measured by determining the shade of grey to which 

 it appears equivalent. Even the most saturated colours (for 

 instance, yellow and blue) have different degrees of brightness. 

 Colourless sensations include the grey series from the deepest black 

 to the most blinding white. 



If a ray of sunlight is allowed to pass through a prism, it is 

 decomposed by its passage into rays of different colours, which are 

 called the colours of the spectrum; they are red, orange, yellow, 

 green, blue, indigo, and violet. The red rays are the least turned out 

 of their course by the prism, and the violet the most, whilst the other 

 colours occupy in order places between these two extremes. The 

 differences in the colour of the rays depend upon the rapidity of 

 vibrations producing each, the red rays being the least rapid 

 and the violet the most. In addition to these, there are other rays 

 which are invisible but which have definite properties ; those to the 



