Septembee 16, 1898.] 



SCIENCE. 



371 



four hundred dollars. The holder is expected 

 to give twenty hours a week to the work of the 

 laboratory, and to devote the remainder of the 

 time to original investigation in histology or 

 embryology under the supervision of the senior 

 officers of the department. Applications should 

 be addressed at once to Professor Charles S. 

 Minot, Harvard Medical School, Boston. 



DISCUSSION AND C0BBE8P0NDENCE. 

 PEE- COLUMBIAN MUSIC AGAIN. 



To THE Editor of Science : The question of 

 pre-Columbian stringed instruments of music in 

 America comes up again, this time from Carl 

 Sapper, the distinguished geologist in Coban, 

 Guatemala. He had learned of my former letter 

 on the distribution of the musical bow and con- 

 cedes with regard to the Loltun Hool, of 

 the Mayas, that it was surely introduced from 

 Africa, since the Kekchi call it marimbadie, 

 or caramba. The same instrument is in use 

 among the Xicaques, in Honduras, but they at- 

 tach a guacal as a resonator. Dr. Sapper does 

 not agree with me that the stringed musical in- 

 strument was entirely absent from the western 

 hemisphere, for, says he, the Lacandones have 

 a two-armed guitar, which he thinks not to 

 have been borrowed. The Kekchi also, says 

 Sapper, uses strings on the scraping instru- 

 ment, called 'su.' This is entirely new to 

 me. As to the double-necked guitar, Mr. E. 

 H. Hawley says that they were common in 

 Europe and may be seen in collections. These 

 have the necks projecting from the same end 

 and parallel or slightly diverging. The Fans 

 have a variety in which the two necks start 

 from opposite sides of the body. One example 

 is made from the stem of a palm leaf 55 inches 

 long. Four strings are cut from the outer skin, 

 their ends being left attached. A little way 

 from the middle a stick is set up perpendicular 

 to the palm stem. On one margin of this are 

 cut four notches or steps, about half an inch 

 apart, to receive the strings. Braided bands 

 of palm fibre encircle stem and strings, and by 

 moving these the latter are tuned. Opposite 

 the upright stick or bridge is tied an open 

 gourd for resonator. I should be glad to re- 

 ceive descriptions of these Central American 



instruments or drawings. Most of all, would I 

 like to examine specimens. If by the scraped 

 instruments Dr. Sapper means some modifica- 

 tion of the notched fiddle, then he has found a 

 prize, but not necessarily a pre-Columbian one. 

 O. T. Mason. 



A curious optical illusion connected with 

 AN electric fan. 

 A curious illusion connected with an or- 

 dinary two-winged pendant fan, such as are 

 commonly employed in restaurants, barber 

 shops, etc., attracted my attention some years 

 ago, and lately, upon my return to the same 

 place, was just as evident as formerly. Very 

 much at a loss for an explanation, the phe- 

 nomenon was described to one of our leading 

 psychologists and educators, but no satisfactory 

 explanation was obtained. Hence, it is sup- 

 posed that possibly the phenomenon has not 

 been noticed by others, and is described here 

 for the benefit of those concerned and with the 

 hope of drawing out similar observations by 

 others. 



The illusion consists in the fan appearing to 

 rotate in the opposite direction from the real 

 one. Sitting some thirty feet away and look- 

 ing at the fan, which is moving at a moderate 

 speed, it is plainly seen to be moving in the 

 direction opposite to that of the hands of a 

 watch. The plane of rotation appears to be 

 horizontal. But as one continues looking the 

 vanes suddenly seem to move in the opposite 

 direction and the plane of rotation to change 

 so as to incline towards the observer. The 

 change is under the control of the will and may 



