Jeff, thank you so much for your help and advice on my question about uniform identification. You might like to know that someone visited my blog the other day (where I had posted that mystery picture) and left a comment identifying my mystery man. According to this reader, the picture was of US Admiral George Dewey.

Once I read that, I checked it by Google images, and found several other copies of the same pose, which was probably taken near the time of 1899.

By teamwork, several heads being better than one, mystery solved!