200 SOME CURIOSITIES OF VISION. 



methods of experimenting were employed. In the first, colored light 

 was obtained by passing white light through colored glasses; in the 

 second and more perfect series of experiments, the pure colored light 

 of the spectrum was used. Among other results, it was found that 

 ceteris paribus the recurrent image was much stronger with green light 

 than with any other, and that when the excitation was produced by 

 pure red light, however intense, there was nd recurrent image at all. 



I intend to attempt a repetition of my first experiment before you. 

 A metal disk with a small circular aperture near its edge is placed in 

 the lantern, and its image projected upon the screen. When the disk 

 is turned slowly the spot of light upon the screen goes round and round, 

 and some of you may, perhaps, be able to see at once that the bright 

 primary spot appears to be followed at a short distance by a much feebler 

 spot of a violet color, which is the recurrent image of the first. It is 

 essential to keep the direction of the eyes perfectly steady, which is not 

 an easy thing to do without practice. (See fig. 2.) If now we place a 

 green glass before the lens, the ghost will be at its best, and all of you 



should be able to see it, provided that you do 



not look at it. With an orange glass the 

 ghost becomes less distinctly visible, and its 

 color generally appears to be bluish-green 

 instead of violet as before. When a red glass 

 is substituted, the ghost completely disap- 

 pears. If the speed of rotation is sufficiently 

 high, the red spot is considerably elongated 

 during its revolution, and its color ceases to 

 be uniform, the rear portion assuming a light 

 bluish-pink tinge. But however great the 

 speed, no complete separation of the spot into 

 red and pink portions can be effected, and no recurrent image is ever 

 formed. 



The spectrum method of observation can only be carried out on a 

 small scale, and can not be exhibited to an audience. It, however, 

 affords the best means of ascertaining how far the apparent color of the 

 recurrent image depends upon that of the primary, a matter of some 

 theoretical interest. I found that white light was followed by a violet 

 recurrent image; after blue and green, when the image was brightest, 

 its color was also violet; after yellow and orange, it appeared blue or 

 greenish-blue. On the other hand, when a complete spectrum was 

 caused to revolve upon the screen, the whole of its recurrent image 

 from end to end appeared violet; there was no appearance of blue or 

 greenish-blue at the less refrangible end. For this and other reasons 

 it was concluded that the true color was in all cases really violet, the 

 blue and greenish-blue apparently seen in conjunction with the much 

 brighter yellow and orange of the primary being merely an illusory 

 effect of contrast. (This contrast effect was illustrated by a lantern 



