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"Polo Team, Durland's Riding Academy, 1908: Cpt. Dr. Fielding Robeson, N.W. Rose, Archer Kinney, Lawrence & Simon Ottinger, Mgr. Trusttum, Gerald & Victor Meyer." 8x10 glass negative, G.G. Bain Collection. View full size.
That is an arena ball, for indoor polo. It is quite a bit larger than the one used on a field, and is soft. The hard ball was made of wood (the word "polo" comes from the name of the hard root which originally was the source for the balls' wood) back then, today of high-impact plastic.
I know nothing about polo, but is that a regulation 'ball'?
Looks like the photographer and big tripod-mounted camera can be seen in the mirror right over the horse whose head is turned. As a photojournalist 100 years later I still have to watch myself and occasionally do get caught!

Look at the size of that picture behind the seating gallery! I wonder if any of these gents went on into the cavalry for WWI. I believe a major (NBC?) broadcast company is in the building now.
[Since 1949, the Durland's building at Central Park West has been home to ABC, at 7 West 66th Street. The sets for "20/20" and "Good Morning America" used to be in what was the main riding ring. - Dave]
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