242 PRACTICAL PHYSIOLOGY 



EXPERTMRNT V. Let the head rest in a fixed position, as by placing 

 the chin in a tin mug, and place a sheet of white paper vertically in 

 front of it at a distance of eighteen inches. Put a dot in the centre of 

 the paper Close one eye and with the other fixate the dot. Take a 

 thin strip of white card-board and blacken about two millimetres of the 

 end. Move the blackened end over the region of the field of vision 

 corresponding to the blind spot, and note the points where the black 

 area disappears, marking them on the white paper. A sufficient 

 number of these points can be taken, nnd a curve drawn through them 

 will indicate the margin of the field of the blind spot. 



4. The Yellow Spot. The experiments performed to exhibit the 

 retinal circulation have shown that there is a certain region in the 

 direct line of vision where the retinal blood-vessels are not visible. 

 This region is coloured by a pigment which absorbs the blue and green 

 of the spectrum, and therefore appears of a reddish-yellow colour and 

 is called the yellow spot. 



EXPERIMENT. Take a flat-sided bottle containing a fairly strong 

 solution of chrome alum, or use a sheet of purple or violet gelatine. 

 Look with one eye closed through the coloured medium at a bright 

 white surface. A rose-coloured oval spot will appear in the centre of 

 the field. The pigment of the yellow spot absorbs the blue and green, 

 and transmits the rest, and hence the predominant red tinge imparted 

 to the area corresponding to the macula hi tea. 



5. Acuteness of Vision in different Regions of the Eetina. In order 

 to differentiate similar objects grouped closely together it is necessary 

 that these should subtend an angle of a certain magnitude with respect 

 to the eye. To be more precise, the angle subtended is at the nodal 

 point of the schematic eye, and this angle again is equal to that sub- 

 tended at the nodal point by the image of the differentiated objects on 

 the retina. In order that objects be differentiated it is apparently 

 necessary that their contiguous margins and the space between should 

 form an image on the retina, which is of certain length. Helmholtz 

 found that a subtended angle of 63 '75", equivalent to a retinal distance 

 of -00463 mm, was necessary for discrimination. As far as this 

 method of investigation is concerned it appears to connect visual acuity 

 with the distribution of the cones. 



[ADVANCED EXPERIMENT. Set up in a good light the parallel line 

 diagram used in the experiment on chromatic aberration (Experiment 

 III ). Or arrange a series of five black wires, separated by their own 

 diameter, against the sky. Walk backward from either of these 

 objects till they can just be no longer discriminated. Calculate the 

 size of the retinal image. 



