362 W. LeConte Stevens—The Stereoscope, etc. 
mage, observed by divergence of the visual lines, with very 
ce real objects visible above the stereoscope, such as a 
nite chain of mountains, the stereoscopic image appears to 
much more remote than real objects the most distant.” The 
ar el anomaly of binocular vision without convergence of 
tion produced with that w 
which is not distinguishable from it but by feebler convergence, 
So. fa 
Helmholtz’s observation ; nor is it sustained by those whom I 
have tested, every one of them giving results closely accord- 
ant with my own, care having been taken to prevent any pre; 
vious knowledge of my object in sense ie them. All that 
is essential is to secure axial divergence and compare the 
binocular effect with the monocular effect of the same picture, 
if the original landscape be not present. Before me is a 
stereograph. representing Alpine spre which I combine 
binocularly, with from 2° 17’ to 2° 40’ f divergence, as fore- 
ground and background are pecan te regarded. On clos- 
ing the left eye, the apparent distance of a remote mountain 
- is not perceptibly diminished; indeed on acconnt of the de- 
ereased brightness of the monocular image, the mountain 
seems slightly farther. ‘To eliminate physical papers as 
s possible, this being always strong in pictures of land- 
scapes, a stereograph is now taken, representing a white marble 
statue against a dark background ; the stereographic interval 
can be varied at will, the card having been cut in two. Plac-. 
ing this in the stereoscope, the two pictures are drawn apart 
until 5° of axial notes is attained, the experiment being 
made at a window from which an extensive landscape can be 
seen for the purpose of comparison. By no effort of imagina- 
tion can I estimate the apparent distance of the statue to be 
more than 10™. A stereograph representing a skeleton cone is 
now Salwticated. but with the same result. 
at normal vision with both eyes, all 
the elements of physical perspective being the same in bo 
ases. is ts probably what eant. ut his remark is 
not necessarily or generally applicable when stereograph and 
landscape are unrelated. Mere divergence of axes is not enough 
to reverse physical perspective, but may modify it to some 
extent and introduce special illusions 
[To be sadlanaty: 
