Hitherto we have alluded only to symmetry of form, but it is manifest that before the union of two similar forms can give pleasure to the eye, there must be also a symmetry of light. If the object =M N O P=, Fig. 15, is white, and its image _m n o p_ black, they cannot possibly form, by their combination, an agreeable picture. As any considerable difference in the intensity of the light will destroy the beauty of the patterns, it becomes a matter of indispensable importance to determine the position of the eye, which will give the greatest possible uniformity to the different images of which the picture is composed. It has been ascertained by the accurate experiments of Bouguer, that when light is reflected perpendicularly from good plate glass, only 25 rays are reflected out of 1000; that is, the intensity of the light of any object seen perpendicularly in plate glass, is to the intensity of the light of its image as 1000 to 25, or as 40 is to 1. When the angle of incidence is 60°, the number of reflected rays is 112, and the intensities are nearly as 9 to 1.—When the angle of incidence is 87½°, the number of reflected rays is 584, and the intensities are nearly as 17 to 10, so that the luminousness of the object and its image approach rapidly to an equality. It is therefore clear, that, in order to have the greatest uniformity of light in the different images which compose the figure, the eye must be placed as nearly as possible in the plane of both the mirrors, that is, as near as possible to the angular point.—But as it is impracticable to have the eye exactly in the plane of both mirrors, the images formed by one reflexion must always be less bright than the direct object, even at the part nearly in contact with the object. The second and third reflexions, etc., where the rays fall with less obliquity, will be still darker than the first; though this difference will not be very perceptible when the inclination of the mirrors is 30° or upwards, and the eye placed in the position already described. It is a curious circumstance, that the positions of the eye which are necessary to effect a complete union of the images—to represent similar parts of the object and its images—to observe the object and its image under the same angular magnitude, and to give a maximum intensity of light to the reflected images—should all unite in the same point. Had this not been the case, the construction of the Kaleidoscope would have been impracticable, and hence it will be seen how vain is the attempt to produce beautiful and symmetrical forms from any combination of plain mirrors in which this position of the eye is not a radical and essential principle. CHAPTER V. ON THE EFFECTS PRODUCED UPON THE SYMMETRY OF THE PICTURE BY VARYING THE POSITION OF THE OBJECT. Having ascertained the proper position of the eye, we shall now proceed to determine the position of the object.