The Relation of Light Perception to Colour Perception. 459 



Thus it will be seen that if, for instance, the reading on the left drumhead 

 is 5320 and that on the right drumhead is 5950, the region of the 

 spectrum from X 5320 to X. 5950 is exposed to view in the eye-piece.* 



The instrument is used as follows : — As far as possible a known quality 

 and intensity of light should be employed. A small oil lamp is quite 

 suitable for the purpose. The observer should first ascertain the exact 

 position of the termination of the red end of the spectrum, the left-hand 

 shutter being moved across until every trace of red just disappears. The 

 position of the pointer on the left-hand drum is noted, and the wave-length 

 recorded. The left drum is then moved so that the shutter is more towards 

 the middle of the spectrum. The right-hand drum is then moved until the 

 pointer indicates the wave-length recorded as the termination of the red 

 end of the spectrum. The observer then moves the left-hand shutter in and 

 out until he obtains the largest portion of red which appears absoh;tely 

 monochromatic to liim, no notice being taken of variations in hrightness, but 

 only in hue. The position of the index on the left-hand drum is recorded. 

 The left-hand shutter is' then moved more towards the violet end of the 

 spectrum, the right-hand shutter being placed at the position previously 

 occupied by the left-hand shutter. In this way the whole of the spectrum 

 is traversed until the termination of the violet end of the spectrum is finally 

 ascertained with the right-hand shutter. The variation of the size of the 

 patches and the termination of the spectrum with different intensities of 

 light can be noted. 



The instrument can also be used for ascertaining the exact position and 

 size of the neutral patches in dichromics, the position of greatest luminosity, 

 and the size and extent of pure colours. When it is used to test colour- 

 blindness, the examinee should first be shown some portion of the interior 

 of the spectrum, and then asked to name the various colours which he 

 sees. In this way he will have no clue to the colours which are being 

 shown him. 



Tested with this instrument a normal individual will, as a rule, name 

 six distinct colours (viz., red, orange, yellow, green, blue, violet), and will 

 mark out by means of the shutters about 18 monochromatic patches. 

 Occasionally we come across individuals with a greater power of differentiating 

 hues, to whom, as to Newton, there is a distinct colour between the blue 

 and violet, which Newton called indigo. Such individuals will mark out a 

 greater number of monochromatic patches, from 22 up to 29. The limited 

 number of monochromatic patches which can be marked out in this way is 



* This spectrometer has been constructed for me by Adam Hilger, Ltd., and was 

 purchased with a grant made by the C4overnment Grant Comuiittee of the Eoyal Society. 



