24 



PRACTICAL PHYSIOLOGY 



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 distinguishable. Measure 

 and record the angles. Draw a section of the eye and mark upon it 

 the parts of the retina which react to black and white, red, green, 

 yellow and blue. 



Study the more elaborate instrument, the Perimeter, used for 

 making an accurate map of the visual filed. 



(Read these subjects in Text Book.) 



6. How are Colours Perceived ? 



(Revise your knowledge of the physical nature of colour. Study 

 the spectrum produced by a prism. 



i. How are the various Colours in Nature produced ? 



METHODS. (1) 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, intro- 

 duce, 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 con- 

 clusions 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) an- 

 other part of the spectrum. 



ii. 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) Fix the special black and white disc provided on the wheel and 

 rotate at various speeds. (Fig. 18.) Note the effects produced. 

 What conclusion do you draw ? 



iii. Can all Individuals distinguish Colours equally well ? 



METHOD. (1) Take a set of Holmgren's wools. Give a student a 

 red wool and let him pick out all that are of the same sort of colour. 

 Find if any member of the class is colour blind. 



FIG. 18. 



