Cited by W. McDougall, 'Some New Observations in Support of Thomas Young's Theory of Light and Colour-Vision', Mind 10 (1) (1901), pp. 52-97, 210-245 and 347-382.
Description:

[note on pp. 355-356]: A piece of apparatus, which may he called a portable dark chamber, is very useful in the study of after-images. It consists of the following parts: a piece of 1 inch plank, 18 inches in width, and 2 feet 6 inches in stands vertically upon this plank, its lower edge is let into a slot running along the middle of it, and it is held firmly in position by angle-pieces. A little above its centre, the square board has a circular hole, 3 ½ inches in diameter. At the top comers of the board a pair of light wooden rods are attached by hinges and can be fixed so as to project horizontally from it, parallel to one another, and at right angles to the plane of the board. Two pieces of black cloth, the inner one of black velvet, the outer of black linen, each about 3 ½ yards in length, are tacked by one edge to the top and sides of the square board and spread over the horizontal rods. A strip of cloth about 1 yard wide tacked by one edge to the base-board and a back-flap of black velvet complete the walls of the chamber. The base-board being placed upon a table at one edge the observer sits within the curtains with his eyes at the level of the hole in the vertical board, and by sitting upon the free ends of the curtains can completely exclude all light. On the inner surface of the vertical board, above and below the circular hole, are two pairs of projecting ledges to hold the photographic shutter and the glass plates. Where no dark room is available this simple piece of apparatus can be made to serve many of the purposes of one. It can also be used as a ' Dunkel-Tonne,' and even where a dark room is available this dark chamber presents the advantage that it can be placed so as to receive the direct rays of the sun through the hole in the vertical board. I have made a second example with two apertures about 9 inches apart in place of the one, each being filled with shutter and glass-plates. This is in many ways a great improvement. I find the Thornton-Pickard shutter for time and instantaneous exposures, well suited for use in such a chamber.' (355-356)