COLOUR- VISION AT DIFFERENT POINTS OF THE RETINA. 231 



single beam of light, in which state they enter the eye. The second part of 

 the instrument consists of an arrangement by which a beam of light from the 

 very same source is weakened by two reflections from glass surfaces, and enters 

 the eye alongside of the beam of compound colours. 



The instrument is formed of three rectangular wooden tubes, the whole 

 length being about nine feet. It contains two prisms, two mirrors, and six 

 lenses, which are so adjusted that, in spite of the very different treatment to 

 which the two portions of a beam of light are subjected, they shall enter the 

 eye so as to form exactly equal and coincident images of the source of light. 

 In fact, by looking through the instrument a man's face may be distinctly 

 seen by means of the red, the green, or the blue light which it emits, or by 

 any combination of these at pleasure. 



The arrangement of the three slits is made by means, of six brass slides, 

 which can be worked with screws outside the instrument ; and the breadth of 

 the slits can be read off with a gauge very accurately. 



In each observation three colours of the spectrum are mixed and so adjusted 

 that their mixture is so exactly equivalent to the white light beside it, that 

 the line which divides the two can no longer be seen. 



It is found that in certain cases, when this adjustment is made so as to 

 satisfy one person, a second will find the mixed colour of a green hue, while 

 to a third it will appear of a reddish colour, compared with the white beam. 



But, besides this, it is found that the mixed colour may be so adjusted 

 that, if we look directly at it, it appears red, while if we direct the eye away 

 from it, and cast a sidelong glance at it, we see it green. The cause of this 

 is the yellow spot, which acts somewhat as a piece of yellow glass would do, 

 absorbing certain kinds of light more than others ; and the difference between 

 different persons arises from different intensities of the absorbing spot. It is 

 found in persons of every nation, but generally stronger in those of dark com- 

 plexion. The degree of intensity does not seem to depend so much on the 



