Trans. N. Y. Ac. Sci. 118 Feb. 13, 



the moon at the horizon, attributing the judgment of increased dis- 

 tance to aerial perspective, as it is viewed through a thicker bed of air. 

 This is unquestionably one of the most important elements, but 

 Brewster« specially denies its value in the present case. Dr. R. 

 Smith made a series of estimates, sometimes toward the horizon, 

 sometimes toward the zenith, and found that the distance of the hori- 

 zon appeared from three to four times greater than that of the zenith. 



Helmholtz' gives reasons why the celestial vault should appear 

 flattened, even though there is no such limiting surface to the space 

 overhead. The path of the visible moon is referred to this imagined 

 semi-ellipse ; and to this circumstance, combined with aerial perspec- 

 tive, is mainly to be referred the illusive judgment of variation in its 

 distance. 



This illusion is by no means confined to our estimation of the dis- 

 tance of heavenly bodies. Most persons have probably observed the 

 apparent magnification in distance of the ground when viewed from a 

 lofty window compared with that of the window when viewed from the 

 ground. In this case aerial perspective can scarcely be considered, 

 and there is nothing to produce the illusion of a geometrically regular 

 surface below. It seems highly probable that physiological rather than 

 mathematical conditions are operative in producing the illusion." 



Mr. W. Le Conte Stevens exhibited 



A new reversible stereoscope. 



The objects to be attained in constructing this stereoscope were — 



I. To secure ready motion to the semi-lenses, so that they may be 

 adapted in position for any pair of eyes, whatever may be the interocu- 

 lar distance, and for any stereograph, whatever may be the stereogra- 

 phic interval within the usual limits. 



II. To secure ready motion to the screen, so that the whole stereo- 

 graph, or either separate half of it, may be visible to each eye, at will. 



III. To secure the possibility of removing the semi-lenses, so that 

 they may be reversed in relative positions, or be substituted by prisms 

 with their bases toward each other, so that the left and right pictures 

 jnay be simultaneously viewed, without discomfort, by the right and 

 left eyes respectively, thus securing reversion of relief in the binocular 

 image, if desired. 



IV. To secure the means of examining the binocular image either 

 alone or attended by monocular images, so that the difference between 

 the two kinds of vision may be noted. 



V. To secure the means of using the same instrument, either with 



°. The Stereoscope. London, 1856, p. aoi et seq. 

 '' . Optique Physiologique, p. 800 tt seq. 



