Beccaria made use of a paper kite in some experiments on electricity. Round this kite appeared a small shining cloud varying in size; the same brightness was even observed round part of the string. Sometimes it disappeared, and if the kite moved faster the light appeared to float to and fro for a few moments on the place before occupied. This appearance, which could not be explained by those who observed it at the time, was the image which the eye retained of the kite relieved as a dark mass on a bright sky; that image being changed into a light mass on a comparatively dark background. In optical and especially in chromatic experiments, where the observer has to do with bright lights whether colourless or coloured, great care should be taken that the spectrum which the eye retains in consequence of a previous observation does not mix with the succeeding one, and thus affect the distinctness and purity of the impression. 31. These appearances have been explained as follows: That portion of the retina on which the dark cross (29) was impressed is to be considered in a state of repose and susceptibility. On this portion therefore the moderately light surface acted in a more lively manner than on the rest of the retina, which had just been impressed with the light through the panes, and which, having thus been excited by a much stronger brightness, could only view the grey surface as a dark. 32. This mode of explanation appears sufficient for the cases in question, but, in the consideration of phenomena hereafter to be adduced, we are forced to trace the effects to higher sources. 33. The eye after sleep exhibits its vital elasticity more especially by its tendency to alternate its impressions, which in the simplest form change from dark to light, and from light to dark. The eye cannot for a moment remain in a particular state determined by the object it looks upon. On the contrary, it is forced to a sort of opposition, which, in contrasting extreme with extreme, intermediate degree with intermediate degree, at the same time combines these opposite impressions, and thus ever tends to a whole, whether the impressions are successive, or simultaneous and confined to one image. 34. Perhaps the peculiarly grateful sensation which we experience in looking at the skilfully treated chiaro-scuro of colourless pictures and similar works of art arises chiefly from the _simultaneous_ impression of a whole, which by the organ itself is sought, rather than arrived at, in _succession_, and which, whatever may be the result, can never be arrested. [1] Plate 1. fig. 1. [2] See the Life of Benvenuto Cellini, vol. i. p. 453. Milan edition, 1806.--T. III. GREY SURFACES AND OBJECTS. 35.