VISION 121 



in the centre of vision and remain for some little time, 

 but will gradually become less distinct. 



The pigment of the yellow spot absorbs the blue-green 

 rays between the lines E and F of the spectrum, these 

 rays removed from those passing through the chrome 

 alum, viz. red and greenish blue, leave a rose colour. 



Or, 6. Look through a sheet of blue glass at a white sheet of 

 paper for 5 to 10 sees., the glass being 10 to 15 cm. from the eyes; 

 then shut one eye, keeping the other open for two or three seconds, 

 and so alternately. In the middle of the visual field a small dark 

 patch often rosy at first will be seen owing to the greater ab- 

 sorption of the blue rays by the pigment' of the j^ellow spot. Around 

 the dark patch, a lighter, larger area will be seen. 



14. Colour vision in different parts of the retina. 



A rests his chin on a. block in the centre of the circle of 

 a perimeter and looks steadily, say, with the left eye, 

 straight before him at a mark on the centre of the cross- 

 ing arms (or axis of the arm) of the perimeter 1 . 



B stands on ^4's left; he has discs -75 cm. in diameter 

 of white, red, green, yellow and blue. 



B taking a black screen in one hand covers the pos- 

 terior part of the left-hand quarter circle, slips the red 

 disc inside the circle at 65, exposes it for about two 



1 A simple form, sufficient for the purposes of this section, is easily 

 made. A hoop is taken made of thin wood, about 5 cm. broad and with 

 a diameter of about 65 cm. ; it is cut in half, and the pieces are fixed one 

 in a horizontal, the other in a vertical plane near the top of a vertical 

 stand about 12 cm. high, and the inner surfaces blackened. Or the ex- 

 periment can be made still more simple. The observer ( ,4) stands over 

 a table on which is a strip of black cloth. He rests his forehead on a 

 block about 30 cm. from the cloth and looks straight down on it. B 

 places the coloured discs on the cloth at various distances, uncovering 

 them for a moment. 



