THE PRINCIPLES OF BIOLOGY. 
45 
The essential elements in stereoscopic pictures are, of 
course, that they should be taken from two points of view not 
less than 2J inches asunder, which is the average distance of 
eyes from centre to centre ; and that they should be fixed 
upon the card so that there is the same distance between 
them, point for point. If they are taken from points of view 
less distant than 2J inches, the proper effect of relief will not 
be produced. If that distance is much exceeded an exagger¬ 
ated relief will be the result. In taking views of distant objects 
this exaggerated relief is always given, because without it no 
stereoscopic effect would be perceived. Our natural eyes 
being only 2| inches apart, are not able to see round the 
corners of any object which is more than 30 or 40 feet distant 
from us. In taking stereoscopic views of objects or land¬ 
scapes more distant than this, the points of view are more 
widely separated in order to get the effect which the eyes 
alone could not get, and no doubt this unnatural perspective 
makes the resulting picture in the stereoscope still more 
surprising and wonderful to us. 
The instrument itself is a very simple but very ingenious 
contrivance. Its object is to refract the light-rays from two 
pictures so that they shall appear to the eyes to emanate from 
one only. This is done by taking a common double-convex 
lens, cutting it across the centre into two halves, and turn¬ 
ing these two halves round so that their thin edges come 
together and their thick parts are turned outward, right and 
left. The two halves are then cut circular or square, and 
fixed into the frame, and the result is that the two pictures 
appear to overlie one another just as if the eyes were con¬ 
verging upon one object. 
THE PRINCIPLES OF BIOLOGY. 
BY HERBERT SPENCER. 
Exposition of Chapters XII. and XIII. 
Multiplication of the Human Race. 
BY ALFRED HILL, M.D., F.I.C. 
The inverse variation between the generative function and 
the preservation of the individual, or, as Mr. Spencer other¬ 
wise expresses it, between Genesis and Individuation, is 
universal. Man’s very slow rate of multiplication—lower 
