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Page:Popular Science Monthly Volume 70.djvu/471

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467
ILLUSIONS OF VISION

spicuous by longer fixation of the gaze upon the colored spot, To observe this, half-inch disks of red, yellow, green and blue paper were pasted vertically on ends of long needles and placed in strong lamplight at a distance of eight feet from the eye. After long unwinking gaze at one of these disks, until general color sensitiveness seemed to be disappearing and the color of the disk itself seemed to be spreading out around it, a quick closing of the eye, or the mere placing of a sheet of paper close before the open eyes, revealed a very interesting succession of changes, as follows:

1. A black or dark green disk with a limited red margin filling the intensified zone, limited by the dark halo. This effect lasted for a very brief instant of time, like the common positive after-image.

PSM V70 D471 Illusion of the position of a mote in the eye.png
Fig. 3. 'Dot' Mote outside the Yellow Spot. Fig. 4. 'Dot' Mote in Yellow Spot but not in Fovea.

2. The outline soon reappeared; the red disk and all white objects taking a dark indigo-blue color, the remainder of the field being a bright yellow. This effect might last a minute or two.

3. During the height of this effect a negative halo appeared for a time around the dark after-image of the disk at the usual distance of 7'. The success of this experiment depends largely upon steadiness of vision and avoidance of winking. The determination of the effect of different colors and conditions offers a fine field for investigation.

The next test with a view to locating the cause of this halo phenomenon was made on motes that so often float by the line of vision. This was done by looking at a highly-illuminated area through a small pin hole held close to the eye. Three classes of motes were observed: First, the usual cell fragments and groups; second, rapidly moving objects probably of similar character, and, thirdly, minute black dots which from their motions seemed to be located in the same region as