44 
PASSAGES FROM POPULAR LECTURES. 
to the same point at the same time. It may be easily con¬ 
ceived that if both eyes are turned to the same point, so that 
there is on each retina a precisely similar picture, the two 
impressions coinciding in the brain may be received as one. 
But in looking at any solid object the two pictures formed in 
the eyes are not precisely similar ; the left eye sees a little 
more of the left side of the object, and the right eye a little 
more of the right side of it. Each eye sees a little way 
round the corner on its own side, so that there is a slight 
difference in the two pictures. What, then, does the brain do 
in this case? Instead of making a confused image of the 
whole, it combines what is alike in the two pictures, and adds 
on the surplus at each side, the result being an impression of 
thickness , as well as of length and breadth, of relief, of com¬ 
parative distances, of perspective. 
Shut one eye, and you will find that everything seems 
flatter, more like a painted picture than when you see it with 
two eyes. A person suddenly deprived of the use of one eye 
feels at a loss about the distances of objects at first, but the 
brain soon learns to work under its altered conditions, and 
after a few days’ experience the sense of perspective is nearly 
as strong as before. 
When you look at a landscape with the head inverted 
(looking under the arm or between the legs), you say “ It is 
like a picture ! ” What you mean is that the perspective has 
in a great measure disappeared. The foreground has gone 
back, and the background has come forward. Everything 
looks nearly in the same plane, and the effect is strange and 
rather pretty. The reason is that the brain is not accustomed 
to estimate distances with the eyes in that position,-and can¬ 
not do it completely on a sudden demand. If you thought it 
worth while to remain upside down for a week you would find 
the perspective come back again, and your pretty picture fade 
away into a real landscape. 
On this principle of binocular vision Sir David Brewster 
founded his invention of the stereoscope, one of the most 
beautiful and interesting of scientific toys. Scenes and 
objects which are far removed out of our sight it sets before 
us with life-like roundness and reality. It makes artificially 
upon the retinae of the eyes the two slightly different pictures 
which the right and left eyes would make for themselves if 
they had the real object before them. The brain does its 
work regardless of the deception. What is alike in the two 
impressions is combined into one, the surplus is added at 
each side, and the effect of relief is produced as perfectly as 
if the object itself had sent its light-rays into the eyes. 
