If the book is not held in  
the frontal plane, the angle  
formed by the rays from its  
endpoints will be compressed,  
the retinal image of this  
surface of the book will thus be  
smaller, and its shape will be  
distorted. The more the  
orientation of the object  
departs from the frontal plane,  
the more its shape on the  
retina will be distorted. In the  
language of perspective, this  
compression is termed  
foreshortening. 
 
    These same principles  
explain why roads, railroad  
tracks, and other parallel lines  
in the environment that are  
not in the frontal plane will 
 yield projections to the eye  
that converge. They also  
explain why an image of  
pebbles on a beach or some  
other uniform texture on a  
surface becomes increasingly  
dense with receding distance.  
In these cases, the visual  
angles subtended by objects and  
by the separation of spaces  
between objects become  
increasingly small as distance  
increases.