the design moved in a circular manner on the screen, and in this case also there existed the illusion that every circle seemed turning round its centre. And what is the explanation of these curious effects? Mr. Thompson does not believe (and we share his opinion) that the faculty possessed by the retina of preserving images during a certain time (_persistence of impressions on the retina_) can entirely explain these phenomena. Without desiring to formulate a decided theory, Mr. Thompson is of opinion that we may class these facts with others which have been known for some time, and that perhaps it is necessary to attribute to the eye some new faculty which may explain the whole at once. [Illustration: Fig. 137.—Another figure of Mr. Thompson’s. The different circles appear to turn round if we give the design a rotating movement.] Brewster and Adams have described phenomena which are equally curious, the principal of which we will describe, adding also some analogous investigations due to Mr. Thompson. The result seems to be that there exists in the eye a badly-defined purpose of nature, which in a certain way _compensates_ (Brewster) for the real phenomenon, because it has a contrary effect, which will continue for some time after the cessation of the phenomena, and which gives by itself a sensation contrary to that which the real movement would have produced. Thus, after having fixed our eyes for two or three minutes on a rushing waterfall, if we suddenly turn our glance on the adjacent rocks, the latter appear to move from top to bottom. It is not a question here of the effect of the relative movement to be observed on regarding _simultaneously_ the falling water and the rocks; if one can succeed in abstracting oneself to such an extent that the water appears motionless, the rocks appear to take a contrary movement. In the phenomenon we describe there is no simultaneous comparison; the eyes are turned _successively_ first on the water, and then on the rocks. In a rapid river, such as the Rhine above the fall at Schaffhausen, the stream is not equally swift in every part, and the current is noticeably more rapid in the middle of the river than near the banks. If we look fixedly at the centre of the stream, and then suddenly turn our eyes towards the banks, it will appear as though the river were flowing back towards its source. This kind of _compensation_ does not only produce an apparent displacement, but also changes in size. When travelling at great speed in a railway train, the objects of the surrounding country as one flies by them gradually appear smaller and smaller. If, when this occurs, we suddenly remove our eyes to the interior of the railway carriage, and fix them on immovable objects, such as the sides of the compartment, or the faces of our travelling companions, the images on the retina will really preserve the same size, and yet the objects will appear larger.