330

Centrum nach Peripherie habe ich jedoch hier nicht beobachten können).

Other experiments were made with sunlight in direct vision and also "morning light" through a milk glass plate. The image of the sun, after direct fixation for about 0.5 minutes showed on a dark field these changes: A small image of bluish white color, surrounded by greenish yellow, and this in turn surrounded by a purple red border. Then the greenish yellow overflowed the blue, which, however, reappeared through the yellow. This was followed by the overflow of the purple red over the whole image. Eventually the yellow reappeared in the center, and out of the yellow came light blue. This process was repeated several times. Then appeared on the border dark blue, which spread over the entire image, and out of the blue reappeared the purple, then yellow, and lastly light blue. (Eine Farben von der anderen noch eingefasst oder sie vollständig verdrängend, wobei wohl auch ein oder das andere Bildchen einmal ausblieben kann). The series ended with black, which appeared as an indefinite shape surrounded by dirty yellow. The duration of this type of after image was estimated to be five minutes or more.

The sequence of colors in the after image of "morning light" seen through a milk glass was greenish yellow, surrounded by purple red and a reddish field. Then followed a well-saturated yellow, surrounded by purple and a bluish gray field. The purple then filled the center, which was followed by well-saturated blue, pale greenish blue, reddish yellow, in which blue would appear as a tiny spot.

Homouth states the chief results of his experiment as follows: (1) The colors of the after image are not connected with each other through gradual transitions, but appear somewhat sharply separated from each other in the visual field. (2) The qualities in these discrete stages of the Abklingen are, for all the stimuli, approximately, the same three, and these are blue, purple, and yellow.

Miles (41) devoted a section of his article on the "Formation of Projected Visual Images" to a brief discussion of the color changes in the image. The subjects were asked to report the appearance of the "fixated cross " in the image and the color of the disc upon which the cross appeared. He used as stimulus the light from an incandescent mantle exposed for two seconds. All work was carried out with bright adapted eye. He reported that his "records emphasize the fact that, in visual experiences, individual differences are most marked." Color changes for some subjects were very scanty," and "in these cases the image was of short duration." For others, there

"

was considerable play of very vivid and distinct colors. "This seems to show that a regular color sequence for light of any particular physical intensity is dependent on personal characteristics, and is not a resultant of the absolute intensity of the light." One subject gave reports of a regular sequence, blue, green, red. Another gave blue, red, "with yellow appearing as frequently as green." Other subjects "show but little evidence of any predominating sequence of color." In connection with the duration of the image, he noted that some subjects "appeared to show very little variation in the duration of the image from test to test." Others began with long duration images and steadily decreased, as the observations were continued. A third type "showed the reverse effect." In one case

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no image was seen after the first two exposures. Then followed, at successive tests, images which lasted 47, 125, and 168 seconds, respectively."

Stevenson (64) performed some experiments at the University of Cincinnati on the negative flight of colors from a white light. He used a strong Nernst lamp as a stimulus. "The observer looked into the dark box from which all light had been excluded. At a signal, the experimenter raised the shutter, allowing the subject to look directly at the stimulus for different periods, from one fourth of a second to thirty seconds, and then the shutter was dropped." "When an image was seen in the dark box, the shutter was raised and the image projected on the white screen. Thus, for each image recorded in the dark box, one on the white screen was recorded." His summary of results in part is as follows: (1) The order of colors and the duration of the flight depend upon the intensity and the duration of the stimulus. (2) The negative flight can only be observed when alternated with the positive and soon fades away when projected upon a white surface. (3) The negative lasts longer than the positive. (4) The positive colors can be seen in a dark box when the eye is in full light of a room, but the flight does not last as long, nor is it as definite as when the eye is in complete darkness. (5) The first reports of most subjects are definite for the middle phases.

The essential features of this review of the literature from the beginning of the nineteenth century to the present time may be conveniently shown in tabular form. The following tabular summary shows the type of stimulus used, together with the details of the color sequence in the image:

Summary of the reports made on the sequence of colors in the after image of a bright light, from the beginning of the 19th century to the present.

Observer

Stimulus

Sequence of colors in the after image

Goethe

dark.

White paper in Brightness, yellowish, purple, blue, bright sunlight.

Brightness, bright red, dark, followed by a brighter center. Becomes gray.

Spectra with the eye open

Pink, orange, yellowish brown, yellow, red, orange. Spectra with the eye closed

Green, blue, bluish pink, blue, indigo.

White, light blue, light green, dark red, dark blue.

Bright blue, violet, dark yellow or yellowish-green.

Green, blue, violet, deep red.

Series I

Green, blue, violet.

Series II

Yellow, orange, red, violet, blue,

green.

Sun directly viewed. Positive series

red, violet, blue, green, yellow.

Sunlight through Green, yellow-green, yellow, orange,

red, scarlet, crimson and brown, or olive.

White, greenish blue, indigo blue, violet or rose-red, grayish orange, dirty yellow-green.

White, blue, green, red, blue (on dark ground) and (on white ground) finally blue-green and yellow.

White, light blue, light green, yellow, red, yellow, white, blue.

Sunlight through Greenish blue, purplish violet,

violet red, pale pink, dull yellow, olive green.

Sunlight through Yellow-green, dull orange, pink,

1 According to Brewster the eyes were alternately opened and closed during the experiments. It has frequently been pointed out that the effects of a secondary light upon the image is to cause it to shift towards the later stages of the color transformation. Further the vision was monocular during fixation. The effect of monocular vision on the color changes has not been carefully investigated up to the present time.

2 This is the series which Miss Washburn designates as the "Maximum series" derived from the maximum duration and the maximum intensity of light stimulus (sunlight).

3 Scott apparently took this to be the normal flight of colors on the authority of some previous writers.

SUMMARY OF LITERATURE

In the course of development of the literature on this subject the following results among others have been obtained:

1. The flight of colors has been obtained from different types of light stimuli. These are in part listed by Aubert as follows: The sun (Newton, Fechner); a bright flame (Purkinje); sunlight on white paper (Fechner); concentrated sunlight on white paper (Fechner); sunlight on white paper viewed through dark tube (Brewster); sunlight through window of dark room (Bonacursius, Fechner, etc.); electric spark (Seguin, Aubert). In addition to these, there have been the later observers who have used sunlight either reflected or transmitted. Homouth used a Nernst lamp.

2. The sequence of the colors in the after image has varied from one observer to another. Only in one or two cases has there been close agreement.

3. Several observers agree that the colored after image varies with intensity and duration of the stimulating light. Fechner, Helmholtz, and Washburn agree that the phenomena are constant after a certain duration and intensity are reached, but below these the images vary in content and duration.

4. Goethe, Fechner, Seguin, Helmholtz, and Homouth agree that under certain circumstances the colors of the image involute from the periphery towards the center. Homouth and Seguin assert that the colors in the image of a very bright light do not always pass from periphery to center, but may appear in the center. Helmholtz asserts that the colors in the image when the stimulus is a moderate illumination appear to be irregular in the manner of their appearance.

5. Brücke and Scoresby assert that objects under low illumination do not give colored after images. On the other hand, Seguin asserts that the image of a white object is always colored. Very few experiments with low illumination have been reported.

6. The doctrine of the cyclical character of the color transformation in the after image has been advanced by McDougall. The cycle is a recurrent one of three colors, green, red, and blue. McDougall is not supported by the other observers in this opinion.

7. Washburn and Titchener have asserted the uniformity of the phenomena for the practised observers. Helmholtz also remarked: "There appears to be no essential difference among different persons in respect to the course of the after images of intense lights which have been developed under the same conditions. So far as they go my observations agree with those of Fechner and Seguin." On the other hand, Fechner, Seguin, and Plateau recognized the variability of the phenomena for different observers, at the same time insisting on the necessity of investigation of the invariable as well as the variable elements.