120 



THE POPULAR SCIENCE MONTHLY. 



tongues of the Antarctic Sea, preserving in 

 the main its characteristic temperature. 

 The explanation of this seems simple. For 

 some cause or other as yet not fully under- 

 stood, evaporation is greatly in excess of 

 precipitation in the northern portion of the 

 land-hemisphere, while in the water-hemi- 

 sphere, and particularly in its southern por- 

 tion, the reverse is the case : thus one part 

 of the general circulation of the ocean is 

 carried on through the atmosphere, the water 

 being raised in vapor in the northern hemi- 

 sphere, hurried by upper-wind currents to 

 the zone of low barometric pressure in the 

 south, where it is precipitated in the form of 

 snow or rain, and welling thence northward 

 in the deepest channels, on account of the 

 high specific gravity dependent on its low 

 temperature, it supplies the place of the 

 water which has been removed. 



A New Form of Etercoseopei — It is 



known to many people that, by placing the 

 axes of the eyes parallel, it is possible so to 

 see stereoscopic pictures without any in- 

 strument as if we were looking at them 

 through stereoscopic lenses. To do this, 

 we may make a small hole in the centre of 

 each picture, and hold the paper in such a 

 position that each eye looks through the 

 hole at a distant object. But with ordinary 

 stereoscopic pictures this object needs to 

 be very far off, so that this contrivance is 



control of his eyes that, without looking at 

 any distant object, he will be able to place 

 the axes of the eyes parallel. Though 

 either of the above methods would answer 

 for experimental purposes, neither would 

 serve as a substitute for the ordinary stereo- 

 scope ; for if one requires such an incon- 

 venient arrangement, the other requires too 

 much trouble in explaiuirg. But I thought 

 that, perhaps by some happy, simple con- 

 trivance, we could see stereoscopic pictures 

 unaided by any lenses, yet without any con- 

 scious straining of our eyes ; and after re- 

 peated experiments I found out that, if cor- 

 responding parts of two pictures were apart 

 from one another only one inch and a half 

 or so, and if by means of a partition the 

 two pictures were so separated from one 

 another that the right eye will see only the 

 right picture and the left eye the left pict- 

 ure, the two pictures will combine just as 

 easily as with an ordinary stereoscope. 



The following is my explanation why 

 two pictures combine so easily when cor- 

 responding parts of the two are apart only 

 one inch and a half or so, while to make 

 ordinary stereoscopic pictures combine re- ■ 

 quires so great effort : 



To combine ordinary stereoscopic pict- 

 ures unaided by any lenses, the axes of the 

 eyes must be placed parallel ; but, since it 

 is not an habitual position of our eyes to 

 have their axes parallel, this can only be 



not a very convenient one ; nor is it de- 

 sirable to make a hole in the centre of every 

 picture we wish to see stereoscopically with 

 our naked eyes. Another method is to make 

 one familiar with the muscular sensibilities 

 of the eyes according as their axes converge 

 or diverge, and to make him acquire such 



accomplished by a great effort. But, if to 

 place the axes parallel requires such an 

 effort, to make them too convergent requires 

 equally great effort. Thus to see a single 

 image of a finger put on the tip of our nose 

 is very difficult, and to see a single image 

 of the tip of the nose itself is almost an im- 



