168 Professor William Stirling [May 12/ 



as well as to uncoloured objects. As the impression lasts about one- 

 tenth of a second, its disappearance is sometimes spoken of as death 

 of an impression. One of tlie easiest ways of showing this is by 

 means of a rotating disk with alternate white and black sectors, or 

 one or more white or coloured spots on a disk, made to revolve with 

 sufficient rapidity. 



Rotating disks of this kind were first described by Muschenbrock 

 in 1700. A heavy metal disk, about 6 inches in diameter, mounted on 

 a vertical axis, moves about sis revolutions per second. In order to 

 get fusion there must be four to sis sectors, black and white, of the 

 same breadth, arranged alternately on the disk. 



Persistence of Vision. 



One of the earliest recorded experiments in physiological optics, 

 that of persistence of vision, is said to be mentioned by Ptolemy in 

 his work on Optics in a.d. 140. " If a sector of a disk be coloured 

 in patches at various distances from the circumference, and then 

 rapidly revolved, the sector will present the appearance of a series of 

 coloured rings." The phenomenon of " after-images " was known to 

 Alhazen in the twelfth century, and to Leonardo da Yinci in the 

 fifteenth century. Hallam, in his "' Introduction to the Literature of 

 Europe," claims " the right of Leonardo to stand as the first name of 

 the fifteenth century." His unpublished MSS. " contain discoveries 

 and anticipations of discoveries within the compass of a few pages, so 

 as to strike us with something like the awe of preternatural know- 

 ledge." 



After-images were also known to that strange "curiosity of 

 Hterature," mathematician, astrologer, eccentric medical man of the 

 sixteenth century, Jerome Cardan, who — according to Bayle and 

 Henry Morley — when he passed through London was consulted 

 about the health of Edward YL, to cast his horoscope and foretell 

 his fate. The King died a few months afterwards in spite of the 

 astrologer's forecast of a longish life. 



He was more successful when he visited St. Andrews in 1550 to 

 attend Archbishop Hamilton, who is said to ha\ e been cured of some 

 affection of the chest by this strange forerunner uf that still stranger 

 and most singular personality Theophrastus Bombast von Hohen- 

 heim, better known as Paracelsus. The invention of the zoetrope is 

 even attributed to Cardanus. 



Comparison of Muscle and Ketina. 



We know that when a muscle is stimulated with an adequate 

 stimulus it contracts. If, however, a sufficient number of stimuli per 

 second are applied to the muscle — directly or indirectly — there is a 

 fusion of the contractions, and the condition known as Physiological 

 Tetanus is set up. There is a striking analogy between a muscular 



