SENSE OE SIGHT. 



871 



fore, see the wall with a dark spot upon it. These accidental colors are 

 produced by fatigue of the retina, and consist of all the other colors of 

 the spectrum except that of the luminous object at which we have been 

 looking. The primary colors, so-called, are red, yellow, and blue. The 

 intermediate points of the spectrum are made up by combinations 

 of these; thus, violet is a mixture of red and blue, green of yellow and 

 blue, and orange of red and yellow. Blue is thus the complement of 

 orange and yellow of violet. 



Fig. 389.— Diagram illustrating Irradiation. (Stirling.) 



If this diagram is held gome distance from the eyes, especially if not exactly focused, the white dot will 

 appear larger than the black, though both are exactly of the same size. 



When rays of light from a strongly luminous body fall upon the 

 retina the}- are not confined exactly to the precise points upon which 

 they impinge, but extend themselves to a greater or less degree around 

 them. Thus, if we make a circular white spot upon a black ground, and 

 a black spot of corresponding dimensions upon a white ground, the 

 former will appear considerably larger than the latter, apparently, be- 

 cause the excitation of the retina by the luminous impression tends to 

 spread itself over the adjacent unexcited space (Pigs. 389, 390, and 391). 



The same phenomena are seen when 

 the experiment is performed with 



Fig. 390.— Diagram illustrating Irra- 

 diation. (Stirling.) 

 The two white squares appear the larger, and they 

 appear to run into each other, and to be joined by a 

 white strip. 



Fig. 391.— Diagram illustrating Irra- 

 diation. (Stirling.) . 

 The white strip, which is of equal width through- 

 out, appears wider below, between the black squares, 

 than above. 



different colored bodies. If the impressing body, though small in size, 

 illuminate the retina strongly, it may irradiate its impression upon the 

 surrounding part of the retina and make itself appear larger. 



In our perceptions of the nature of external objects we find that 

 the sense of vision, like the other sensations, is not infallible, but that 



