TRANSACTIONS OF SECTION D. 781 



visible only Ly transmitted light, and best seen in the most healthy condition of 

 the eye, has hitherto hindered the complete explanation of either. 



*The two kinds of images, of the same corpuscles, can be accurately superposed 

 by means of alteration in the curvature of the retina, produced bv the action of 

 the rectus muscles of the eyeball in the manner above described, which is apparent 

 to consciousness as a strain on the eyeball. 



*Pressure images and superposed normal images, arranged in a number of 

 circular whirls in the area of the fovea centralis, show that the so-called non- 

 vascular area of the retina is furnished with a highly complex system of capillary 

 loops. These loops, which like the n^ajor part of the retinal capillary system, run 

 vertical to the plane of the retinal layers, are apparently derived from the deeper 

 or arterial plexus, which belongs to the ganglionic layer. They are doubled back 

 upon themselves, so that the corpuscles in the looped end are seen in the form of 

 a rapidly turning star as tliey turn round it : the reason of this arrangement is 

 that there are here no venous branches to receive them, the venous or outer 

 plexus, belonging to the inner nuclear layer, having ceased with the boundarv of 

 the fovea centralis, in the manner which is apparent in most injected preparations, 

 and which formerly gave rise to the belief that the fovea centralis was a nocr- 

 vascular area. 



*5. Light-dots: these are minimal units of pressure stimulation which must 

 not be confounded with what the Germans call ' light-dust,' the latter consistino- 

 (as appears from the description given by Ilelmholtz) in confused pressure imao-ea 

 of blood-corpuscles, and being therefore made up of shining spots which are^at 

 once less distinct and much larger. These light-dots, which are constantly visible- 

 and uniformly distributed over the whole of the field of vision, governed by 

 movements uniform in character but capable of ditl'erent directions, are apparently 

 due to the pressure of the blood-current on the smallest structures of the retina, 

 either the constituents of the molecular layers or units of the laver of rods and 

 cones. The units are so small that tlie pressure image of a blood-corpuscle would 

 cover a number of them, together with the greatly larger intervals between them. 

 It is probable that the visibility of light-dots is largely a question of temperament ; 

 nevertheless they appear to be an important factor" in the production of optical 

 illusions of motion. 



6. Optical Illusions of Motion ; covflicting theories referred to the test of 

 certain hitherto mulescrihed eyitoptical plienomena. By Beatrice 

 Lindsay, Girton College, Gamhridge. 



The chief examples of optical illusions of motion are the following: 1. Hecti- 



linear, usually horizontal illusion of motion, the typical instance of which is the 

 apparent backward movement perceived by passengers in a train which is going- 

 through a tunnel ; 2. Centrifugal illusion of motion, by which stationary obi ecte 

 seem to expand after looking at an object diminishing in the distance ; 3. Rota- 

 tory illusion of motion (Thompson's strobic wheels) by which stationary concentric 

 circles appear to rotate round their common centre. 



, , ^ -• 1 , - , -.lally accepted 



theory ; tlie iormer is the theory supported, and I think substantiated, by the tacts 

 here adduced. 



The chief objections to the former theory are as follows :— L The first illusion 

 is supposed to be sufficiently accounted for by the opposite rush of neo-ative 

 images described by Brewster as occurring when a train of movin"- imao-es 

 suddenly ceases. To this it may be replied that these negative images only occur 

 when the eye is fatigued and not in its normal condition; furthermore, the opposite 

 direction of the rush of negative images requires in itself to be accounte 1 for 

 2. With regard to the centrifugal illusion, it is supposed impossible for the retina 

 to move in all directions at the same time, so as to produce an ' all-round ' illusion 

 To this it may be replied that a very complex curve of illusion can be produced by 



