In the Conservatoire des Arts in Paris a visitor will always notice a number of people looking at the mirrors in the “optical” cabinets. These mirrors deform and distort objects in a very curious manner, and people find much amusement in gazing into them till they are “moved on” by the attendants. Such experiments create great interest, and a very excellent substitute for these may be found in a coffee-pot or even in a large spoon, and all the grotesque appearance will be seen in the polished surface. The least costly apparatus will sometimes produce the most marvellous effects. Look at a soap-bubble blown from the end of a straw. When the sphere has a very small diameter the pellicule is colourless and transparent; but as the air enters by degrees, pressing upon all parts of the concave surface equally, the bubble gets bigger as the thickness decreases, and then the colours appear,—feeble at first, but stronger and stronger as the thickness diminishes. The study of soap-bubbles and of the effects of the light is very interesting. Newton made the soap-bubble the object of his studies and meditations, and it will ever hold its place amongst the curious phenomena of the Science of Optics. But before going into all the phases of Lights and Optics we will proceed to explain the structure of the eye, as it is through that organ that we are enabled to appreciate light and its marvellous effects. [Illustration: Fig. 92.—Grotesque effects of curved surfaces.] It is often considered an embarrassing matter to fix precisely the value of two lights. Nothing, however, can be easier in reality, as we will show. In comparing different lights, it is necessary to bear in mind the amount of waste, the colour of the light, the luminous value of the source, and the steadiness of the flame. The luminous value of a lamp-burner is generally equalled by that of a wax candle, and we will take as an example one of those at six to the pound. Very precise appliances are used for this experiment when great exactness is required; but it is easy to calculate in a simple manner the differences in ordinary lights. Supposing we desire to test the value of light given by a lamp and a wax candle, they must both be placed on the table at an equal height, B and A, (fig. 93), in front of some opaque body, A, and then a large sheet of paper must be fixed as vertically as possible to form a screen. When B and A are lighted, two shadows, E and F, are produced, to which it is easy to give exactly the same intensity, by advancing or withdrawing one of the two sources of light. The intensities of the two lights will then be inversely proportional to the squares of the measured distances, AB and AC. By a similar careful calculation it has been possible to draw up a table of the relative values of various ordinary lights. We have not included here the electric light, which has recently attracted so much attention, because this system of lighting can hardly be said to