ILLUSIONS OF VISION 



467 



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 lamp- 

 light 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 spread- 

 ing 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. 



Fig. 3. 'Dot' Mote outside the 

 \ello\v 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 phe- 

 nomenon 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 



