W. Le Conte Stevens—Physiological Optics. BST 
means of double images, and that the mind cnstinetively distin- 
guishes between those made by objects that are respectively 
farther and nearer than the point fixed. his last proposition 
would be hard to demonstrate experimentally, but even when 
the attention is not specially given to the double images, these 
may, and probably do, play an important part as elements in 
the unconscious formation of judgments. Again it has been 
stated* that we see ai long and short viistances ut the same time, 
because the retina has thickness and transparency, and images 
are focalized at different depths beneath its surface. Here 
again we can neither affirm nor deny in answer, although re- 
cognizing the fact that the erystalline lens, being one of short 
focal length, has in consequence of spherical aberration consid- 
erable “depth of focus.” It is moreover fluorescent, irregular 
in structure, and imperfectly centred. Perfectly sharp focali- 
zation is hence impossible, as shown in the radiated appear- 
ance of stars and the irradiation about any brilliant surface like 
that of the crescent moon. These optical defects may be in 
some respects advantageous in ordinary vision. If : 
theory be true, though not demonstrable, it may partly explain 
the possibility of binocular combination when the differences 
between the two pictures are so minute that the perception of 
double images in any part of the binocular field is impossible. 
Some idea can be formed of the minuteness of the stereoscopic 
displacement actually necessary when we consider that Mr. 
Warren De la Rue succeeded in obtaining a stereograph of the 
sun, from which by stereoscopic vision, the ridges of the faculze 
could be perceived in sharp relief. On the stereograph of the 
moon, to which reference has been more than once made, the 
levation of mountain ranges and solitary peaks, and even the 
inequalities of the supposed dead sea bottoms can be clearly 
seen. The crater Copernicus and the lunar Apennines stand 
forth particularly boldly, and the ridge that divides the bed of 
the heart-shaped “Sea of Serenity” can be easily traced. Any 
one who has undertaken the preparation of a stereograph with 
the pencil or pen knows how very difficult it is to avoid the 
a naar of roughness in the combined image at places where 
smoothness is desired. No two impressions from the same 
mathematical meaning has been assigned to this expression, and 
obviously none can be. It is generally thought, but has not 
n C 
* Towler, The Silver Sunbeam, p. 310; KE. & H. T. Anthony, 1879, 
