October 7, 1898.] 



SCIENCE. 



479 



filled. Among the minor University appoint- 

 ments during the past year were the following : 

 Mr. Shaw, Emmanuel, Assistant Director of 

 the Cavendish Laboratory ; Mr. H. W. Pear- 

 son, Christ's, Assistant Curator of the Her- 

 barium. To University lectureships — Mathe- 

 matics, Mr. Love, St. John's ; Midwifery, Mr. 

 Stabb, Downing. The following have been ap- 

 pointed demonstrators : Mechanism and Ap- 

 plied Mechanics, Mr. Peace, Emmanuel ; Ani- 

 mal Morphology, Mr. J. G. Kerr, Christ's ; 

 Botany, Mr. R. H. BiflBn, Gonville and Caius ; 

 Pathology, Mr. T. Strangeways Pigg ; teacher 

 in Anthropology, Mr. Duckworth, Jesus. 



The chair in the medical department of the 

 University of Pennsylvania, vacant by the death 

 of Dr. William Pepper, will not be filled at 

 present, it being recommended by the faculty 

 that, for the present, Dr. James Tyson, profes- 

 sor of clinical medicine, be given full and gen- 

 eral direction of the department of medicine, 

 and that four assistants. Dr. John H. Musser, 

 assistant professor of clinical medicine, Dr. Al- 

 fred Stengel, Dr. M. Howard Fussell and Dr. 

 Frederick A. Packard, instructors in clinical 

 medicine, be appointed to deliver, under Dr. Ty- 

 son's supervision, didactic lectures on medicine. 



Miss Agnes M. Claypole, Ph.B., Buchtel 

 College and M. S. , Cornell University, has been 

 appointed assistant in microscopy, histology and 

 embryology at Cornell University. 



Secretary Long has directed that the course 

 in naval architecture begun at the Naval 

 Academy at Annapolis last year under the now 

 famous Constructor Hobson be transferred to 

 the Massachusetts Institute of Technology. 

 The course will cover a period of three years, 

 including practical instruction in the summer 

 in ship yards and navy-yards. Eight cadets 

 will be detailed to take the course. 



Mr. Frank Irving Shepherd, recently act- 

 ing professor of chemistry and physics in the 

 University of Denver, has been appointed in- 

 structor in chemistry in the University of Cin- 

 cinnati ; and Dr. Thomas Evans, formerly 

 instructor in organic chemistry at the Massachu- 

 setts Institute of Technology, and more recently 

 chief chemist of the Proctor & Gamble Soap 

 Company and of the American Cotton Oil Com- 



pany, has been appointed instructor in technical 

 chemistry at the University of Cincinnati. 



Professor Hofer, of the University at 

 Munich, has been appointed professor of geog- 

 raphy in the University at Wiirzburg. Dr. 

 Loewenherz has been qualified as decent in 

 physics in the University at Konigsberg. 



DISCUSSION AND CORRESPONDENCE. 



THE WINDMILL ILLUSION. 



In the issue of Science for September 16th 

 Dr. F. C. Kenyon calls attention to the optical 

 illusion to be seen when viewing a rotating 

 electric fan, and requests some explanation of 

 the same. The illusion consists in the appar- 

 ently capacious reversal of the direction of rota- 

 tion, and in a corresponding change of the 

 plane of rotation. 



This phenomenon has long been known to 

 those who have investigated illusions of motion. 

 So far as the writer's knowledge'goes, it was first 

 mentioned in literature in 18G0, by the Germaa 

 Sinsteden. * Since that time frequent mention 

 has been made of it and several explanations- 

 have been propounded. The essentials of the 

 illusion can be seen in the case of windmills, 

 electric fans, rotating bars, or rotating disks- 

 bearing heavy radial strips of black. For all 

 of these the name of ' The Windmill Illusion' 

 is currently employed. 



The explanation of the illusion is obviously 

 to be sought in the interpretation of equivocal 

 factors entering into the total experience 

 known as the perception of rotation. Sinsteden, 

 and Helmholtz after him, tried to explain the 

 matter along this line. That eye-movemenls do 

 not enter in has been shown by experiments 

 reported by Dr. Nichols at the first meeting of 

 the American Psychological Association. The 

 essence of the explanation lies in the considera- 

 tion that the perception of rotation often rests 

 upon the perception of distinct positions of the 

 rotating body, and that the succession of these 

 various positions or phases of rotation admits 

 of either of two interpretations as re- 

 gards the direction of rotation. That we 

 do perceive motion by means of its phases 

 may be readily demonstrated by rotating a disk 

 bearing variously colored sectors in a dark 



* PoggendorfE's Annalen, CXI., 336. 



