The following Illusions are founded on two well-known facts; namely, the persistency of impressions, and the pro-duction 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 sub-ject than is contained in the first Part.As an apology for the apparent disregard of taste and fi-ne art in the plates, such figures are selected as best serve the purpose for which they are intended.