DANCER'S DISC. 115 



interior, every ring will appear yellow at its outer, and blue at its inner edge ; 

 and if the colours present together very slight differences, we may fall into 

 the illusion which causes the differences really existing between the colours 

 of the different rings to disappear, leaving instead, on a uniformly coloured 

 background, the contrasting blue and yellow of the edges of the rings. It is 

 very characteristic that in these cases we do not see the mixed colours, but 

 seem to see the constituent colours separately, one beside the other, and one 

 through the other. 



All the experiments we have described afford great interest to the 

 student ; they can easily be performed by those of our readers who are 

 particularly interested in these little-known subjects. Any one may 

 construct the greater part of the appliances we have enumerated, and others 

 can be obtained at an optician's. The discs in particular are extensively 

 manufactured, and with great success. 



