As to the optics involved, I will attempt the rationale behind the deepening of the color, not much, but, a shade or two darker. The suns rays striking the particles in the air causes the atmospheric disturbance which changes the color of what you are visualizing. Those images in the distance are dim and blurred. Those images that are closer to the viewer are more sharp and colorful. So, when the tree shades those air particles between the viewer and the object being viewed, there is less atmospheric disturbance to distort what is being seen. Therefore, it will appear a shade or two more pronounced than that which is seen on the perimeter of the object being viewed. This is pretty general but does it make any sense?
Chris, thanks for the tip on the book. I'll have to spend my life's savings and get one.
