116 Sir D. Brewster's Observatiofis on the supposed 



rendered permanently visible. To apply this explanation to the phenome- 

 non in question, Mr. W. observed, that whenever the flame of the candle 

 changes its place, the shadows of the vessels fall on different parts of the 

 retina; which is evident from the motion of the figure while the eye re- 

 mains still, which is always in a contrary direction to that of the flame. 

 Hence the shadow, being thus made to change its place on the retina, re- 

 mains, according to the law above stated, permanently visible ; but instantly 

 the flame is at rest, the shadow also becomes stationary, and consequently 

 disappears. 



" Mr. Wheatstone then exhibited an instrument for showing an original 

 variation of this experiment : it consisted of a circular plate of metal, about 

 two inches in diameter, blackened at its outer side, and perforated at its cen- 

 tre with an aperture about as large as an ordinary gun-hole; to the inner 

 face was fixed a similar plate of ground glass. On placing the aperture 

 between the eye and the flame of a candle, and keeping the plate in mo- 

 tion, so as to displace continually the image of the aperture on the retina, 

 the blood-vessels will be seen distributed as before, but will now appear 

 brighter, and the spaces between the ramifications will be seen filled with 

 innumerable minute vessels, anastomosing with each other in every direc- 

 tion, which were invisible in the former experiment. In the very centre 

 of the field of vision there is a small circular space, in which no traces of 

 these vessels appear. Mr. W. remarked, that the absence of these mi- 

 nute obstructions to light will probably account for the greater distinctness 

 with which small objects are there seen, and also for the difference of co- 

 lour observed by anatomists in that spot of the retina." 



In this experiment Mr. Wheatstone has described, 1st, the 

 common method of seeing the blood-vessels, and 2ndly, an 

 original variation of the experiment by which the blood-ves- 

 sels are seen much more distinctly and completely. 



As Mr. Wheatstone was so kind as to lend me, when at 

 Oxford, his plate of metal, &c, I was enabled to see the very 

 phaenomenon which he saw, and I have repeated the experi- 

 ment fifty times since under many modifications. I have 

 therefore no hesitation in asserting that the ramifications exhi- 

 bited by Mr. Wheatstone's apparatus are not blood-vessels, 

 but are nothing more than the ramifications described in my 

 paper already referred to (Lond. and Edinb. Phil. Mag., vol. i. 

 p. 170. §. 1.). If we throw aside the ground glass in Mr. Wheat- 

 stone's apparatus, and look at a luminous surface through the 

 circular aperture when moved as he describes, the same phae- 

 nomenon will be seen ; and if we substitute a rectilineal aper- 

 ture, and make the line of motion perpendicular, or nearly so, 

 to its longest sides, the phaenomena will be seen still more di- 

 stinctly ; and if we look through one or more narrow slits, as 

 in my experiment, the effect will be the same. In short, the 

 edges of the circular aperture in Mr. Wheatstone's apparatus 

 perpendicular to the line in which the aperture is moved, per- 

 form the part of the rectilineal slit or slits in my experiment. 

 Mr. Wheatstone will have no difficulty in recognising the per- 

 fect identity of the two experiments, and he will therefore see 



