THE SENSES 



119 



central spot of the eye, this change in colour suggests that a 

 chemical decomposition accompanies stimulation. 

 3rd. Electrical changes. (See p. 83.) 



2. The power of localising the source or direction of 

 illumination has now to be considered. Its acuteness 

 may be determined in the same way as in studying the 

 sense of touch by finding how near two points may be 

 stimulated and still give rise to a double sensation. Over 

 the central spot two points of illumination may be as 

 near to one an- 

 other as about 

 four micro-milli- 

 metres, and still 

 two sensations be 

 experienced. This 

 is determined by 

 finding the small- 

 est optical angle 

 which can be sub- 

 tended by, say 

 two stars, with- 

 out their images 

 being fused. This 

 angle varies from 

 seventy-three to 



FIG. 57. The Power of localising the Source of Illu- 

 mination on different parts of the retina. The two 

 points, a-b, subtended by the small angle, fall close 

 together at a-b near the centre of the retina, and 

 still give rise to a double sensation ; but if two 

 points, c-d, have their images formed on the peri- 

 phery of the retina, c-d, these images must be far 

 apart to cause a double sensation. 



fifty seconds in 



different individuals, and this corresponds to from 5 -31 to 

 3*65 micros on the retina (Fig. 57). Over the central spot 

 the centres of the cones are about this distance from one 

 another, and it would seem that, to get a double sensation, 

 two cones must be stimulated. On passing to the more 

 peripheral part of the retina, where the cones are more scat- 

 tered, the power of localising decreases, and larger and larger 

 optical angles must be subtended by the two objects e.g. 

 the points of a pair of compasses in order that both may 

 be seen. (Experiment.) 



3. Colour Sensation Physics of Light Vibration. Phy- 

 sically the various colours are essentially different rates of 



