534 



SCIENCE. 



[N. S. Vol. IV. No. 93. 



dry; the tracings will be permanent and can 

 be photographed or printed directly by trans- 

 mitted light. Some ten years ago I had a top 

 made from an excellent gyroscope by removing 

 the supporting ring and fitting a socket on one 

 arm of the axle, in one end of which(the socket) 

 was a female screw. I also had several ' points ' 

 made of hardened polished steel, one ending in 

 a very fine point, one in a truncated cone -i^ of 

 an inch across the smaller base, others smaller 

 and one in a hemisphere say i of an inch in di- 

 ameter. 



These were made to screw into the socket, 

 and the whole most carefully centered by the 

 very best mechanical skill to be had. It was 

 set in motion as humming tops usually are, 

 with a string and wooden handle. 



I send you a few of my tracings, with the 

 sharp point.* The abrupt changes in direction 

 are due to my tilting the glass, and are always 

 approximately perpendicular to the inclination, 



* These were made so long ago that I cannot be 

 certain whether the sharpest point was used, or one 

 that measured ^rV of an inch across its face. 



but never exactly so. In the tracings which I 

 send you they begin at the larger curve and 

 grow smaller as they progress. In a few cases, 

 very few out of hundreds, this ife reversed. 

 The very small undulations, which are so 

 marked a feature in most of the spirals, are due 

 to minute nutation and precession resulting in 

 the larger effects, as the minute movements of 

 the earth result in the grand precession of the 

 equinoxes. 



Sometimes the smaller movements are so 

 very small that they leave no visible traces. 

 All that is seen is what I may call the second- 

 ary curve. Sometimes even that so nearly dis- 

 appears that the path becomes to the eye a 

 straight line. 



If the glass plate is 'level,' i. e., approxi- 

 mately so, interesting figures are traced, ob- 

 lique spirals I may call them, i. e., spirals traced 

 about a point which is not quite stationary. 



At first glance they appear merely like flat 

 spirals out of center. Looking at one of them 

 steadily, with one eye or both, you look into a 

 deep basket resting on its smaller end. Look 

 a little longer, and without knowing how it 

 happens, the basket is reversed, it rests on the 

 larger end, and you see only the small bottom 

 and the outside. 



Look longer, and without seeing any change 

 you are looking again into the basket. 



Now look at the figure with both eyes, but 

 as if focussed for a distant object. You will see 

 two baskets, and probably both in the same 

 position, i. e., both with the small end, or both 

 with the large end, toward you. Keep look- 

 ing ; move the paper quickly a little in any di- 

 rection, both will reverse ; if you have good 

 luck, with a little practice, you will soon get it. 

 Look a little longer. One basket will stand on 

 the small end, and the other on the large one. 

 Focus the eyes on them or near by, and there 

 will be only one basket. This is all well shown 

 in the short spiral I send you. 



Double images are common enough, but the 

 new and singular thing is that they appear to 

 each eye so different, and that all these 

 changes take place without effort. You do not 

 see them change, but only that they are 

 changed. C. B. Warring. 



September 26, 1896. 



