PHYSIOLOGY 



the fovea. As the periphery of the retinae is approached the number of 

 the cones very rapidly decreases, and we should therefore expect to find a 

 limit to the size of the visual field for different colours. This may be tested 

 by means of the perimeter (Fig. 279) and small coloured discs, or more 

 accurately by suitable apparatus for employing spectral colours. By thesr 

 methods it is found that the colour fields are smaller than those for light, 

 but more or less concentric with them (Fig. 288). The actual si:/e <>l flu- 

 fields varies with the intensity 

 and size of the test light source 

 or object. The order in which 

 the colours disappear varies some- 

 what but appears to be usually 

 as follows: First green, then 

 yellow, then red, and lastly blue. 

 The determination of the size of 

 the colour fields is a technique of 

 considerable practical importance, 

 because they are found to be- 

 come restricted in those progres- 

 sive lesions of the optic nerves 

 which may finally lead to total 

 blindness, and also in inflamnia 

 tory conditions of the retina and 

 choroid. Careful examination of 

 the apparent limits of the blind- 

 spot by means of similar appa- 

 ratus was found byHaycraft to show that there is a similar variation in 

 the relative sizes of the fields for different colours (Fig. 288). The same 

 phenomenon is also found round the blind- spots formed in the retina by 

 disease. 



SIZE THRESHOLD OR VISUAL ACUITY 



If a small source of light be gradually reduced in size, a point is .-,0011 

 reached at which it becomes invisible. If it is a coloured source, it as a rule 

 shows a well-marked photochromatic interval, that is, it first loses its colour 

 and then disappears lator. If the intensity of the source is very groat, tin- 

 . i/c has to be greatly reduced before it becomes invisible: it is liecaiise 

 <>l this that we see the stars. It' the size and intensity at the point of dis- 

 appearance be measured, it is found that when multiplied together they 

 equal a constant, so that as in the case of the light threshold the deter mining 

 factor appears to be the amount of light which falls on the retina. In 

 the case of intermittent illumination a similar relationship is found. 



Visual acuity is the ability to see as separate the images of small bright 

 light-sources of any shape placed very close together. Experiment 

 shows that, the distance between the sources must be increased as their 

 distance from the eye is increased. In other words that the angle which 



Kn;. liSS. Limitation of colour fields round 

 the blind spot. (HAYCBAFT.) 



