SYSTEMATIC SYNTHESIS OF TISSUES INTO ORGANS. 293 



pupil quite back to the black choroid. Hence, light cap 

 readily pass through the retina to the choroid, and all 

 that reaches it is neutralized, absorbed, killed, and there- 

 fore none is reflected ; and that which affects the nervous 

 structure, must do it while passing through the structure 

 or by the reaction of the choroid upon it. 



750. Inf. There must be A SPOT IN THE EYE WHERE THE FIBRES 

 ENTER, where cells and other parts of the retina, except fibres, do not 

 exist ; this spot is devoid of sight, showing that the fibres alone are not 

 sufficient for sight, and that cells are necessary. 



751. Ill-us. Close the left eye, and with the right one look at the 

 left-hand spot, at ordinary reading distance; the right-hand spot will 

 not be seen if the distance is correct ; if seen, move the book nearer or 

 farther, and the spot will vanish, and reappear at a greater or less dis- 

 tance. The reason is, that the light from the spot falls on the blind 

 point in the eye. Close the right eye, and, with the left look at the 

 right-hand spot, and the same effect will be produced. 



752. jRemark. THE NERVOUS STRUCTURE OF THE EYE, the protecting 

 sclerotic, the supplying choroid, and the visual retina of five layers, are 

 now complete for the reception of light. 



Remark. THE RETINA APPEARS TO BE merely a ganglion of peculiar 

 form, adapted to receive the influence of light. 



Nature of Light. 



753. There are THREE PRIMARY KINDS or LIGHT, 

 red, yellow, and blue, by the proper proportional mix- 

 ture of which all shades of all colors can be produced. 

 (PL 8.) 



754. Illus. Three parts red and five yellow, produce Orange, eight 

 parts ; three red and eight blue, produce Purple, eleven parts ; Jive yel- 

 low and eight blue, produce Green, thirteen parts ; while three red, five 

 yellow, and eight blue, produce White, sixteen parts. These parts, colors, 

 and proportions, are the natural ones, those of the rainbow, pure and 



750. What said of? 751. Describe Illns. with spots? T52. What said of ? 

 What does ? 753. What said of ? 754. Illussrate proportions of color. 



