The following Illusions are founded on two well-known facts; namely, the persistency of impressions, and the production of complementary colours on, the retina.
The explanations are divided into two parts. The first consists of directions for seeing the spectres. The second, a brief and popular, as well as a scientific, description of the manner in which the spectres are produced, and is intended for the use of those who may wish to know more of this subject than is contained in the first Part.
As an apology for the apparent disregard of taste and fine art in the plates, such figures are selected as best serve the purpose for which they are intended.
To see the spectres, it is only necessary to look steadily