Deans. NAV ACA Sz. 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. Itseems 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, sothat the left and right pictures 
may 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 
8. The Stereoscope. London, 1856, p. aor e¢ seg. 
7, Optique Physiologique, p. 800 ¢¢ seg. 
