782 REPORT — 1887. 



kno-\vn methods, and that tlie UBion of both eyes to accomplish similar but 

 opposite curves would produce this ' all-round ' illusion. 3. With regard to the 

 rotatory illusion, it is contended that no proof can be given of the existence of a 

 movement in the eye. To this it may be replied that when the rotatory illusion is 

 voluntarily induced, pressure images of blood-corpuscles appear, showing that the 

 retina is being subjected to some peculiar muscular strain. 4. The most important 

 objection made to the theory of movement of the retina has been that the optical 

 illusions of motion cannot be voluntarily induced ; but, with the exception of the 

 centrifugal illusion, I find that they can, by practice, be so induced. 



The voluntaiy production of optical illusions is dependent on the above described 

 curious entoptical phenomenon, that there is constantly and invariably present to the 

 normal eye, when trained to such observations, a current of small uniform pressure 

 excitations, distinguished as such by their metallic lustre, faint yellow in colour, 

 and of minimal size and brilliancy. This current of minimal points of light, here- 

 after indicated by the name of • light-dots,' is, as above stated, apparently to be 

 interpreted as an incessant wave-movement of pressure images, due to the mechanical 

 excitation produced by the circulation of the blood in its passage under or near 

 certain minimal units of the retina, possibly the rods or cones or the peripheral 

 nerve-fibrils respectively connected with each of these. 



This interpretation of the current of ' light-dots,' although it appears startling, 

 eeems to be wai-ranted by the follc>wing data: — 1. The minimal size of the dots, a 

 number of which stand in the area which is filled by the pressure image of a blood- 

 corpuscle. 2. Their uniform size and uniform distribution at fixed intervals 

 throughout the circle of distinct vision. 3. The character of the current as a wave 

 of increased brilliancy affecting fixed points, without any movement of the individual 

 dots. 4. The constant and invariable presence of the phenomenon to the trained 

 eye under all circumstances of accommodation, and all circumstances of light and 

 darkness. 5. The uniform normal rate of the movement. 6. Its unmistakable 

 increase of speed, associated with a greater brilliancy of the dots, under circum- 

 stances which increase the rate of the pulse, such as mental exertion or e.xcitemeut, 

 or brisk physical exercise. 



By practice the current of ' light-dots ' can be made to move in anj' direction 

 desired. When the current is watched or altered its speed increases, a movement 

 of the retinn, probably in the direction of the viovement observed, heiny thereby 

 indicated. In the fovea centralis the ' light-dots ' seem to go round in circles, 

 indicating the complex circulation in that region, which is apparent from other 

 experiments. The possibility of observing this current in all directions tallies with 

 the theory of its connection with the blood-supply travelling in all directions ; its 

 variation "in definite lines indicates that, in watching it, a movement of the retina 

 takes place in a definite direction ; and the ordinary observation of the current, as 

 possessed of a horizontal movement, is probably governed by the easiest line of 

 movement for the eyeball, viz., in a more or less horizontal direction. 



This current of light-dots, with its associated movements of the retina, is shown 

 by experiment to be connected with the conditions of optical illusion in tlie follow- 

 iiio- way. Four horizontal movements of the current may be distinguished, and, 

 after some practice, induced at will, by movements which are not apparent to 

 consciousness otherwise than as a vague sense of effort: namely (1 and 2), 

 movement respectively to the right or to the left at a normal rate ; these are con- 

 ditions of the still and resting eye ; (3) a movement at the increased speed which 

 indicates some movement of the retina in the direction of a moving train of 

 obiects ; this is the condition of the eye watching the landscape from a moving 

 railway-carriage ; (4) a movement, at the same increased speed, in the opposite 

 direction from the mo\'ing train of objects; this condition of the eye, when 

 induced involuntarily by the sudden removal of the train of objects watched, pro- 

 duces an illusion of sudden backward movement, as in the case of the familiar 

 illusion given when a train passes through a tunnel. The same illusion may be in- 

 duced volunt arily in a tunnel or in travelling by night, and either can be voluntarily 

 removed by the" reverse movement of the eye. This fourth condition of the eye, 

 when voluntarily induced by daylight, produces an apparent extra speed of the 



