With a wedge photometer, a small lamp projects an “artificial star” onto a piece of glass, so it could be compared with the star under observation. In use, a wedge of glass was slid in front of the lamp, gradually dimming the light until the two images appeared the same. Charles Pritchard, Savilian professor of astronomy at the University of Oxford, devised the form. Edward C. Pickering, professor of astronomy at Harvard, introduced it to the United States. This example, which came from Princeton University, was apparently made according to Pickering’s design. It may be the one that William Maxwell Reed brought from Harvard to Princeton in 1901, attached to the 23-inch refracting telescope, and used to measure the brightness of variable stars.
Ref: S. P. Langley, C. A. Young, and E. C. Pickering, “Pritchard’s Wedge Photometer,” Memoirs of the American Academy of Arts and Sciences 11 (1888): 301-325.