Vision of the Blood-vessels of the Eye. 117 



that the ramifications are nothing more than the new forms 

 given to the luminous and dark parallel lines produced by the 

 action of light upon the retina. In order to demonstrate this, 

 let us use Mr. Wheatstone's own apparatus. The general di- 

 rection of the ramifications is invariably perpendicular to the 

 direction in which the aperture is moved. If we change this 

 direction from a horizontal to a vertical line the ramifications 

 change their direction also, so that we can give them any in- 

 clination we please. They cannot, therefore, be pictures or 

 representations of any blood-vessels in the eye. 



This unequivocal result would have induced me to believe 

 that the ramifications seen by the common method had a si- 

 milar origin, and were owing to the action of the rectilineal 

 sides of the flame upon the retina, had I not succeeded in 

 seeing this phenomenon with my own eyes. At the Observa- 

 tory of Cambridge, last summer, Sir John Herschel pointed 

 out to Mr. Airy and myself the method by which he saw the ra- 

 mifications, and we were all successful in observing the same 

 phenomenon. This method scarcely differed from that de- 

 scribed by Mr. Wheatstone, but the ramifications which I saw 

 were toto ccelo different from those produced by Mr. Wheat- 

 stone's apparatus : they had not, indeed, one property in com- 

 mon but that of ramifying. The one was seen with great 

 difficulty and occasionally in the middle of a brownish red 

 light, which did not proceed directly from the candle; while the 

 other was distinctly and continuously seen in the middle of 

 condensed light proceeding directly from the candle or other 

 luminous body. 



Regarding, therefore, the phenomenon as real, and the 

 ramifications as occasioned by a blood-vessel of the retina, I 

 shall proceed to examine the different explanations that have 

 been given of it. 



The explanation given by Mr. Wheatstone is exceedingly 

 ingenious ; and the principle which he lays down, and which 

 is printed in Italics, is in every respect well founded. This 

 property of the retina, by which it is unable to maintain the 

 continued visibility of an object seen obliquely, or at a distance 

 from the axis of vision, was communicated by me to the Royal 

 Society of Edinburgh on the 19th of January 1818, and has 

 since appeared in several elementary works on optics ; and it 

 is a necessary corollary from the law of oblique vision, that 

 any movement of the object must restore its visibility b}' re- 

 moving the cause of its disappearance, namely, the continued 

 action of the light upon the retina. 



So far, therefore, Mr. Wheatstone's explanation is unim- 

 peachable; but when he states that the motion of the flame 



