1897.] on Some Curiosities of Vision. 357 



In investigating the influence of colour upon the Young effect, 

 two methods of experimenting were employed. In the first, coloured 

 light was obtained by passing white light through coloured glasses ; 

 in the second and more perfect series of experiments, the pure 

 coloured light of the spectrum was used. Among otlicr 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 no 

 recurrent image at all. 



I intend to attempt a repetition of my first experiment before 

 you. A metal disc with a small circular aperture near its edge is 

 placed in the lantern, and its image projected upon the screen. 

 When the disc is turned slowly the sj)ot 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 apj)ears to be followed at a short 

 distance by a much feebler spot of a violet colour, which is the re- 

 current image of the first. It is essential 

 to keep the direction of the eyes per- 

 fectly 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, pro- 

 vided that you do not look at it. With 

 an orange glass the ghost becomes less 

 distinctly visible, and its colour generally 

 appears to be bluish-green instead of 

 violet as before. When a red glass is 

 substituted the ghost completely disaj^- 

 pears. If the speed of rotation is suf- -pio 2 



ficiently high, the red spot is considerably 



elongated during its revolution, and its colour ceases to be uniform, 

 the rear portion assuming a light bluish-pink tmge. 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 8 

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

 affords the best means of ascertaining hovf far the apparent colour 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 colour 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 colour was in 

 all cases really violet, the blue and greenish-blue apparently seen in 

 Vol. XV. (No. 91.) 2 b 



