What is it?

[MSgtUSMC]

http://i265.photobucket.com/albums/ii220/MSgtUSMCRet/DSC00514.jpg
My son came into possession of this revolver & he asked me what is it. I don't know. Do you?

[k-g]

One of many similar (to me) rimfire revolvers.

http://www.armchairgunshow.com/otsDSZ_spurtriggers.htm

Any marking? Can you measure the chambers and bore for an idea of the caliber?

[k-g]

Well I'll be .....

http://www.earmi.it/armi/atlas/112.htm

Theses pictures are now available in the much revised The Illustrated Encyclopedia of HANDGUNS, by A.B. ZHUCK, Stackpole Books, in English.

[MSgtUSMC]

The word "Eureka" on strap above cylinder. Appears to be .22 rimfire. Cylinder too short for .22 long rifle. Maybe .22 long.

[k-g]

"Hopkins & Allen (Norwich, Connecticut) operated from 1868 to 1915, when they were sold to Marlin-Rockwell. Many of their firearms were distributed under various names."
And they, notably, produced the Merwin & Hulbert models.
Their .22's seem to have been seven-shooters.

(source: Antique Guns by John E Traister.)

[MSgtUSMC]

Found it. It is an Iver Johnson.