664 THE SENSE OF SIGHT. 



lying upon a red surface, a greenish tint ; it has an orange- 

 coloured tint upon a bright blue surface, and a blueish 

 tint upon an orange-coloured surface ; a yellowish colour 

 upon a bright violet, and a violet tint upon a bright 

 yellow surface. The colour excited thus, as a contrast to 

 the exciting colour, being wholly independent of any rays 

 of the corresponding colour acting from without upon the 

 retina, must arise as an opposite or antagonistic condition 

 of that membrane ; and the opposite conditions of which 

 the retina thus becomes the subject would seem to balance 

 each other by their reciprocal reaction. A necessary con- 

 dition for the production of the contrasted colours is, that 

 the part of the retina in which the new colour is to be 

 excited, shall be in a state of comparative repose ; hence 

 the small object itself must be grey. A second condition 

 is, that the colour of the surrounding surface shall be very 

 bright, that is, it shall contain much white light. 



The retina corresponding to the point of entrance of the 

 optic nerve is completely insensible to the impressions of 

 light. The phenomenon itself is very readily shown. If 

 we direct one eye, the other being closed, upon a point at 

 such a distance to the side of any object, that the image 

 of the latter must fall upon the retina at the point of 

 entrance of the optic nerve, this image is lost either 



instantaneously, or very soon. If, for example, we close 

 the left eye, and direct the axis of the right eye steadily 

 towards the circular spot here represented, while the page 

 is held at a distance of about six inches from the eye, 

 both dot and cross are visible. On gradually increasing the 

 distance between the eye and the object, by removing 

 the book farther and farther from the face, and still 

 keeping the right eye steadily on the dot, it will be found 

 that suddenly the cross disappears from view, while on 

 removing the book still farther, it suddenly comes in sight 



