Contributions io the Physiology of Vision. Ill 



of two upper and two lower principal branches, which are va- 

 riously ramified towards the middle of the field of vision, 

 where a dark point is seen, which sometimes appears concave. 

 A similar, but inverted figure is perceived in the left eye ; but 

 to Dr. Purkinje, who is weak-sighted in this eye, it appeared 

 rather irregular and incomplete (fig. 22). The origin of the 

 vessels is a dark oval spot, with a light areola; the figure itself, 

 or rather fragments of it, are seen under various other circum- 

 stances. As was observed above, there can be no doubt that 

 the figure is formed by the central vessels of the retina *. 



* This is an easy experiment to repeat, and is certainly a singularly beautiful 

 one ; the blood-vessels of the retina, with all their ramifications, are distinctly seen 

 projected, as it were, on a plane without the eye, and greatly magnified. I have 

 found the experiment to succeed more perfectly when, the eye being stedfastly 

 dinrtod forwards, the light is made to move right and left below the eye, or up- 

 wards and downwards at the side of the eye ; for when the flame is in the field of 

 view the image is indistinct : the eyelids of the unemployed eye should not be 

 closed, but the light should be obstructed by the hand or any other covering. It 

 is indispensable that the light be in motion, for directly it becomes stationary the 

 image breaks into fragments and disappears : during the motion of the light the 

 imago also moves, and in a contrary direction to that of the light. No image 

 arises when the light moves to and from the eye, nor when it is alternately shaded 

 and uncovered ; the effect, therefore, cannot be attributed to variations of intensity 

 in the light. One of the most remarkable circumstances of this phenomenon is 

 that at the point corresponding to the projection of the foramen centrale, a crescent- 

 formed image is occasionally observed ; its appearance depends on the position of 

 the light with respect to the axis of the eye : for instance, when the light is placed 

 below the eye, the image appears on looking downwards, and becomes obliterated 

 on looking upwards, and in general it appears on looking towards the light, and 

 disappears on looking from it : the mark always appears concave in the direction 

 opposite to the light. That the variable mark just mentioned is in the centre of 

 distinct vision I ascertained by the following experiment : I impressed on my eye 

 the spectrum of a coloured wafer, by looking intently on a black dot at its centre } 

 on causing then the vascular image to appear, I saw the centre of the spectrum 

 coincide with that of the mark. Dr. Purkinje has given no explanation of this 

 phenomenon ; the following is an endeavour to supply the omission. Were the 

 blood-vessels which are spread on the anterior surface of the retina entirely opaquC) 

 they would prevent the transmission of light to the nervous matter beneath them, 

 and their distribution' would be constantly visible; but they are transparent, and 

 in ordinary cases the intensity of the light which passes through them does not 

 materially differ from that which falls directly on the retina. When, however) 

 the retina is fatigued by a strong light, the veins become visible, because the 

 retina is rendered insusceptible to a portion of the light they transmit; but this 

 eflect is only momentary, for those parts which are thus shaded from the more 

 intense light promptly recover their usual susceptibility, and the images vanish I 

 but they may again be made perceptible by displacing them on the retina ; and 

 by making them constantly change their places the images may be rendered per- 

 manent. The momentary appearance of these images may be frequently observed 

 on looking at a strong light immediately after waking in the morning, and may 

 be reproduced several times by successively shutting and opening the eyes. The 

 mark in the middle of the field of vision is most probably a shadow, occasioned by 

 a slight convexity or concavity in the retina at that point. 



The more minute vessels of the retina may be rendered visible in the following 



