TKANSACTIONS OF SECTION D. 783 



train, blurring' objects even when the train is slow. By the voluntary choice of 

 direction and speed for the current of ' light-dots ' many similar illusions may be 

 determined — e.g., the apparent hastening of the moon or stars through clouds, the 

 apparent stationary position, notwithstanding its vibration, of a train in the dark, 

 or on one's waking up from sleep ; the speed of a passing train attributed to a train 

 at rest, &c. In brief, the apparent movement voluntarih/ determined for the current 

 of light-dots defermive.s, modijies, or destroys, according to its relative direction, the 

 occurrence of these illusions. 



The centrifugal illusion appears to Ije similarly induced by a more complex 

 movement of the retina; the rotatory, by watching the circling currents of the 

 fovea centralis. Involuntary continuation of the curved line of movement induced 

 by watching water makes flat surfaces appear to move in wavy lines. Reflex 

 reversal of the current, after long-continued reversal of the direction of moving 

 objects, as at sea, tends to repeat itself when the cause is past, producing by 

 coincidence with respiration, viz., of an upward movement with expiration, and 

 a downward movement with inspiration, the sensation of going down and up in a 

 boat — an illusion commonly experienced after a sea-voyage. 



It must be added that this and other optical ilkusions are probably to be re- 

 garded, not as independent and purely optical phenomena, but as belonging to a 

 complex set of sensations, initiating, in voluntary illusion, or following upon, in 

 pathological illusion, those sub-conscious sensations connected with the ear which 

 are associated with the balance of the body in its various positions. 



7. The demonstration of a new Myorfrajyliion. 

 By Professor McKendeick. 



8. A neiv Physiological Frinciple for the Formation of Natural Bodies. 

 By Professor Jessen. 



9. A neiv Geometry for the Bodies of Man and Animals. 

 By Professor Jessen. 



10. Further siipplementarij remarhs on Supposed Cycloidal Rotation of 

 Arterial Bed Discs.^ By Surgeon-Major R. W. Woollcombe. 



The author suggests that the difficulty of finding with a microscope in bodies so 

 small and symmetrical the existence of cycloidal rotation may be lessened by the 

 attention of the observer being less directed to the rotation itself than to some of 

 its consequences ; thus, even in a gyroscopic disc of four inches it is not always 

 easy to say, even when rotation is rapid, whether there is rotation or not ; but 

 when an attempt is made to impart rotation about a second axis of the disc, then 

 that about the first or .shortest diameter is at once declared by the resulting pre- 

 cessional movement or tendency to rotate about a third axis intermediate to the 

 other two ; so, by examining an artery that is not straight but spiral, a precessional 

 movement in the discs may be found there (?) causer! by the spirality of the artery 

 superimposing a tendency to a rotation about a long diameter on the (assumed) 

 previously existing cj'cloidal rotation about the disc's shortest diameter, and such a 

 compound movement towards a third axis may become visible when the ordinary 

 rotation (as within a straight arter}') might not. Secondly, in the latter the mode 

 of travelling of discs would ))e more orderly than if there were no regular rotation, 

 as the planes of rotation would, by the rotation, be more or less ti.xed, and thus be 

 more parallel to one another than if the discs were devoid of rotation about their 

 .shortest diameter. Thirdly, it might be possible for a skilled manipulator in such 



' Continued from the Brttish'Associaticn Bejoris for 1881 and 1886. 



