On the Causes of Erect and Single Vision, 409 



the mind. There exists indeed a certain space between the 

 two eyes outwardly on the face, but the colouring of this in- 

 tervening space is not painted on either retina, and therefore 

 cannot be noticed by the mind ; nor is there any intervening 

 horizontal purple presented to the mind: Each point of scarlet 

 does but coalesce with each point of scarlet, and each point of 

 purple on one retina, with each point of the similar purple on 

 the other retina ; there is no surplus intervening purple on 

 either retina, and it has been shown that " repetitions of similar 

 impressions of colour do not yield to the mind the sense of 

 an equal number of figures." 



But should the scarlet circle be painted on joints of the 

 retinas which do not correspond, then there will necessarily 

 arise the sense of tivo figures ; because in that case dissimilar 

 impulsions of colour are carried forward to the mind, and 

 yield that proportional variety which determines the obser- 

 vation of conscious duplicates : for the pictures are then im- 

 mediately painted upon different parts of their respective 

 retinae, and the distance between two objects " will be pro- 

 portional to the arch of either retina, which lies between the 

 picture on that retina, and the point corresponding to that of 

 the picture on the other retina*." On this intervening arch a 

 surplus quantity of purple would be interiorly and horizontally 

 painted, and would thence separate the two scarlet figures : 

 Jive impulsions would carry five conscious colours to the mind, 

 and two separated figures would immediately be observed. 



An illustration of these ideas, and especially of this last state- 

 ment of Dr. Reid's, might very easily be imagined, by con- 

 ceiving two small terrestrial globes to be painted with precisely 

 similar colours : Let them be rectified to the same degree of lati- 

 tude and longitude, and similar colours only will appear on the 

 visible surfaces of each ; turn them both so many degrees to 

 the east or west, still only similar colours will present them- 

 selves ; but let one of the globes remain at rest, and turn the 

 other any number of degrees of longitude to the west, a 

 new country will arise to the east on the globe so moved, 

 whose variety of colouring must necessarily prevent the notice 

 of a mere uniformity of colouring on the two globes, and which 

 variety will separate the appearance of. any given country on 

 them, ¥ proportionally to the number of degrees marked on the 

 arch of the horizon" through which the globe so moved had 

 passed. Inspire the colouring on their surfaces with a simul- 

 taneous single sensibility, and it will immediately be perceived, 

 that no consciousness of the existence of any two similar 



* See Dr. Reid's Inquiry, ch. vi. sec. 13. 

 Nexv Series. Vol. 3. No. 18. June 1828. 3 G countries 



