Related to Ernst Mach (trans. C.M. Williams), Contributions to the Analysis of the Sensations (Chicago, 1897 [German ed. Prague 1885]).
Description:

'It is a well-known fact that an optical impression which arises physically later may yet, under certain circumstances, appear to occur earlier. It sometimes happens, for example, that a surgeon, in bleeding, first sees the blood burst forth and afterwards his lancet enter. [note: Compare Fechner, Psychophysik. Leipsic, 1860. Vol. II., p. 433.'] Dvorak has shown, [note: Dvorak, "Ueber Analoga der persönlichen Differenz zwiscben beiden Augen und den Netzhautstellen desselben Auges." Sitzber. d. königl. böhm. Gessellschaft der Wisenschaften (Math-naturv. Classe), vom 8. Marz, 1872.'] in a series of experiments which he carried out at my desire, years ago, that this relation may be produced at will, the object on which the attention is centred appearing (even in the case of an actual tardiness of 1/8-1/6 of a second) earlier than that indirectly seen. It is quite possible that the familiar experience of the surgeon may find its explanation in this fact. The time which the attention requires to turn from one place at which it is occupied, to another, is shown in the following experiment instituted by me. [note: 'communicated by Dvorack, loc. cit.'] Two bright red squares measuring two centimetres across [NB: diagram of squares here] and situated on a black background eight centimetres apart, are illuminated in a perfectly dark room by an electric spark concealed from the eye. The square directly seen appears red, but that indirectly seen appears green, — and often quite intensely so. The tardy attention finds the indirectly seen square when it is already in the stage of Purkinje's positive after-image. A Geissler's tube with two bright red spots at a short distance from one another, exhibits, on the passage of a single discharge, the same phenomenon.' (112-113)

Related to 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:

'Observation X.— l fixed a photographic shutter over an opening in the window-shutter of the dark room and over the former a sheet of rnilk glass, so that when the photographic shutter was opened there appeared on the milk glass a bright white disc about 5 cm in diameter.' (64)

 

'Observation XIV.— I fixated with the right eye the shaded disc, brightly lit by diffused daylight from the outside with white light, for sixty seconds. On closing the shutter there appeared a bright positive after-image. This I projected on to the dark wall beside the shaded disc at an angle of about 15° from it and then opened the shutter again so as to illuminate the shaded disc again.' (69)

 

'I have... made many systematic series of observations on the after-images produced by the fixation of patches of W[hite] light of various intensities and periods of duration and with borders variously defined. I have used various sources of W light... These all give essentially similar
results, and-I will describe only three series in each of which the source of light was a patch of daylight reflected from white clouds upon a sheet of white milk glass. The milk glass was suspended over an aperture in the window-shutter of the dark room, and in the aperture was a photographic shutter which, when closed, completely excluded the light, and when opened allowed the light of the sky to fall upon an area of the milk glass...

I made many series of observations with different degrees of illumination from very bright W to dull grey. For each degree of brightness I made fixations of 1, 2, 5, 10, 20, 40, 80 and 120 seconds, in every case fixating the centre of the disc with both eyes at a distance of about 50 cm., and shutting off the light completely and suddenly at the end of the period by releasing the photographic shutter.' (74-75)

 

'Observation XXVI.—I cut a vertical slit, 30 mm. by 15 mm, in a sheet of cardboard and, at a horizontal distance of 15 cm from it, a second hole in the form of a cross of the same dimensions as the former. This I fitted into an aperture in the shutter of the dark room and covered the one hole with R[ed] paper and the other with G[reen]; so that there appeared a bright R patch and a bright G cross on a dark ground.' (225-226)

 

'Observation XXXII.—I cut a hole 15 mm by 10 mm in a piece of cardboard and fixed the carboard over an aperture in the window-shutter of the dark room, and covered the hole with a piece of the P[urple] paper supplied by Bothe for the colour-wheel. On fixating this bright P patch, with accommodation maintained, struggle occurred between P and G[reen], and with relaxed accommodation I could not observe any change of colour. I then fixed a glass prism vertically before the shutter in such a position that on looking into it I could see the P patch refracted through the prism.' (232-233)

 

'Observation XXXV.—In each case I fixated a dark spot near the centre of the disc, with both eyes at a distance of 18 inches, and then suddenly shut off the light by releasing the photographic shutter.' (238)

 

'Observation XXXVIII.—Sunlight was reflected upon a plate of milk-glass fixed, as in the case of observation x., behind the photographic shutter and covered with S [black] paper save for a circle 5 cm. in diameter. Outside the shutter two sheets of R[ed] gelatine were pinned over the aperture. At a distance of 80 cm. from the circle a sheet of W[hite] paper was pinned against the dark window-shutter (W in figure 21) and illuminated by dull diffused daylight. From a distance of 70 cm. I fixated a point x midway between the W paper and the circle. On opening the photographic shutter the circle, hitherto a dull W, becomes a bright R[ed]. If the shutter be opened in the first few seconds no marked change of colour of W is perceptible, generally only a slight reddening of it through irradiation and reflexion of R[ed] light from my face.' (350-351)

 

[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 | 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)