39 



Now turn the blackboard into the horizontal plane, or use 

 the top of the table, and map out the horizontal field of 

 vision. 



B. Colours. Using coloured papers, map out the field of 

 vision for the different colours, red, green, blue and yellow, 

 noting the points at which the colour becomes clearly dis- 

 tinguishable. Measure and record the angles. Draw a sec- 

 . tion of the eye and mark upon it the parts of the retina 

 which react to black and white, red, green, yellow and blue. 



Repeat these observations using a Perimeter. 



VI. How are Colours perceived? 



Revise your knowledge of the physical nature of colour. 

 Study the spectrum produced by a prism. 



1. How are the various Colours in Nature produced ? 



1. METHODS. Fix a disc of pure spectral colour, e.g. red, on 

 the rotating wheel in a good light, and after rotating the disc 

 and observing it, introduce, by means of the slit, (a) a segment 

 of white, rotate and observe ; (6) a segment of black, rotate and 

 observe ; (c) a large segment of bluey green, rotate and observe. 

 Record your results, and draw conclusions as to the effect 

 upon the colour sensation of mixing a spectral colour with 

 (a) white (diluting it), (6) black (decreasing the illumination), 

 and (c) another part of the spectrum. 



2. Are Colour Sensations produced only by Ethereal Vibrations 

 of different Lengths ? 



METHODS. (1) Insert the tip of the little finger into the 

 external angle of the eye, getting it as far back as possible 

 and turning the eye inwards. Now press, and notice if any 

 colour sensation is produced. 



(2) Make a white cardboard disc with one half blackened, 

 and draw lines as shown in the disc provided. Put a needle 

 through the centre and rotate it as rapidly as possible in a 

 good light, and notice the effect produced. CONCLUSION ? 



