Vision, tracing them to Functional Actions of the Brain. 345 



other eye, to prevent the result from being disturbed by any 

 stronger light entering my left eye. A lighted candle was 

 placed in a dark lantern behind me, with only a small open- 

 ing in it to permit a stream of white light to fall on the slip of 

 white paper S, which was placed so as to be visible to both 

 eyes when they were directed to a distant point P. The ex- 

 periments, with the other primary colours, were made with 

 narrow tubes of thin coloured paper. One of the tubes being 

 applied to the right eye, was strongly illuminated by means of 

 lights placed near its sides; and a black tube was applied to 

 the left eye, to insure thai inequality of the action of the co- 

 loured light on the two eyes, which, even in a more moderate 

 degree, had been found sufficient with 'johite light to produce 

 the phaenomena : the results in all these trials were striking 

 and uniform. The image of S, seen by the eye exposed to the 

 primary coloured light, was constantly of the colour that was 

 complementary to that of the tube or light employed, — an ap- 

 pearance manifestly referrible to the affection of sight mention-: 

 ed in Note *, p. 255, &c. ; but the image of S that was seen 

 through the black tube was uniformly "iVhite, being never in 

 the smallest perceptible degree changed by the affection of the 

 other eye. 



From these observations we learn that no excess of any 

 primary coloured light entering one of the eyes is able to pro- 

 duce the affection which we have been investigating ; hence 

 it follows that 'white light only is capable of exciting it. But 

 the 8th experiment proves that lahite light' also fails to produce 

 it, when it forms a distinct image on the retina. It is not the 

 action of the white light, thei-efore, but the indistinctiiess of 

 the vohite image., that constitutes the true exciting cause. 



This conclusion leads us to remark, that the affection of 

 vision now under investigation, as well as that which is dis- 

 closed under Notes * and f, p. 255 and 257, are both produced 

 by the same exciting cause, or at least by causes of the very 

 same nature. Before inquiring, therefore, into the intimate 

 causes or seat of these analogous affections, it may be of use to 

 comj)are the indistinct images in both with one another, and 

 with their respective effects, in order to detect, if possible, 

 any physical differences between them and their distinct images 

 that may serve to account for the phaenomena they produce. 



When rays of light from a primary coloured object are in- 

 tercepted by the retina before they reach their focal point, the 

 image is rendered indistinct by the diffusion and mixing of 

 rays from single points of the object over many points of the 

 retina. Rays from a "white object similarly intercepted, have, 

 in addition to tiiis cause of indistinctness, another, arising from 



7'hird Series. Vol. 1. No. 5. Nov. 1832. 2 Y 



