118 Sir D. Brewster's Observations on the supposed 



causes the shadows of the vessels to Jail on different parts of the 

 retina, we can no longer follow him. Unless the blood-vessel is 

 placed at a certain distance in front of the retina, and conse- 

 quently in the vitreous humour, it can have no moving shadow; 

 and unless it is within the refracted cone of rays which pro- 

 ceed from the candle, it can have no shadow either moveable 

 or stationary. If the shadow here referred to, be the shadow 

 produced from the direct light of the candle, then the blood- 

 vessel would appear across the visible flame of the candle, and 

 not at the side of it in the reddish brown light. But independent 

 of these objections to the application of the optical principle 

 previously laid down, there are two facts which appear to be 

 conclusive against the explanation : the one is, that the blood- 

 vessels of the retina are not at a distance from it; and the other, 

 that the ramifications may be seen distinctly when the candle is 

 not in motion *. There is one objection more to this explana- 

 tion, which appears to me a formidable one : the ramifications 

 ought to be distinctly and readily seen when the light which 

 forms the shadow is reduced to the same state of dilution, and 

 the same colour, as the reddish brown light on which they ap- 

 pear. This experiment I have repeatedly made with light of 

 all degrees of dilution and divergency, but I have never been 

 able to see a trace of the ramifications. 



If the ramifications in question are the representation of a 

 blood-vessel, it becomes very interesting to ascertain the cause 

 to which their visibility is owing. The first step in the in- 

 quiry is obviously to determine the origin of the reddish brown 

 light in which the phaenomenon is seen. It is quite clear that 

 the brown light is no part of the cone of refracted rays that 

 proceed from the candle : it is equally clear that it is not 

 produced by two or more reflections from the curved surfaces 

 which bound any of the humours of the eye, because in this 

 case it would be of the same colour with the light of the can- 



* The force of this last objection will depend on the circumstances of the 

 case. Mr. Wheatstone says that the image " continues only while the 

 flame is in motion," and that " directly, or soon after, the flame becomes 

 stationary, it dissolves into fragments and disappears." Now if this is a 

 phaenomenon of oblique vision, the image ought not to disappear perma- 

 nently. One part of it should disappear while another part remains visi- 

 ble, and the whole may for a short time continue invisible ; but it will soon 

 reappear, because it would require great steadiness, both in the hand and 

 head of the observer, to keep the shadow on the same part of the retina, 

 though even this would not ensure its permanent invisibility. If this, there- 

 fore, were a phaenomenon of indirect vision, the difficulty would consist in 

 losing sight of the ramifications, whereas the difficulty really consists in 

 seeing them ; and this difficulty is so great with me, that 1 have never been 

 able to see them acain since 1 saw them at Cambridge. 



