Two track levels of P.R.R. on the horseshoe curve, train rounding curve in distance 1 mile away, 100 ft. higher, Pennsylvania, U.S.A.

"The first effect of looking at a good photograph through the stereoscope is a surprise such as no painting ever produced. The mind feels its way into the very depths of the picture. The scraggy branches of a tree in the foreground run out at us as if they would scratch our eyes out. The elbow of a figure stands forth as to make us almost uncomfortable." Oliver Wendell Holmes, an affordable stereo viewer inventor for the American market. Atlantic Monthly, June 1859.