Before I go further into the subject of reflection, let me point out a further mode of dealing with the direction of the light. For instance, here is a candle, and I can employ the principle of _refraction_ to bend and direct the rays of light; and if I want to increase the light in any one direction, I must either take a reflector or use the principle of refraction. I will place this lens (fig. 56) in front of the candle, and you will easily see that by its means I can throw on to that sheet of paper a great light; that is to say, that instead of the light being thrown all about, it is _refracted_ and concentrated on to that paper. So here I have another means of bending the light and sending it in one direction; and you see above a still better arrangement for the same purpose,--one which comes up to the maximum, I may say, of the ability of directing light by this means. You are aware that without that arrangement of glass the light would be dispersed in all directions; but the lens being there, all the light which passes through it is thrown into parallel beams and cast horizontally along. There is consequently no loss of light--the beam goes forward of the same dimensions, and will consequently continue to go forward for five or ten miles, or so long as the imperfection of the atmosphere does not absorb it: and see, what a glorious power that is, to be able to convert what was just now darkness on that paper into brilliant light!
Whenever we have refraction of this sort, we are liable to an evil consequent upon the necessary imperfections in the form of the lens; and Dr. Tyndall will take this lens, and will shew you even in this small and perfect apparatus what is the evil of spherical aberration with which we have to fight. This can be illustrated by means of the electric lamp: if you look at the screen, you will see produced, by means of this lens, a figure of the coal points. This image is produced by the rays which pass through the _middle_ of the lens, a piece of card with a hole in the centre being placed in front; but if, keeping the rest of the apparatus in the same position, I change this card for another piece which will only allow the rays to pass through the _edge_ of the lens, you observe how inferior the image will be. In order to get it distinct, I have to bring the screen much nearer the lamp; and so, if I take the card away altogether, and allow the light to pass through all parts of the lens, we cannot get a perfect image, because the different parts of the lens are not able to act together. This spherical aberration is, therefore, what we try to avoid by building up compound lenses in the manner here shewn (fig. 58). Look at this beautiful apparatus--is it not a most charming piece of workmanship? Buffon first, and Fresnel afterwards, built up these kind of lenses, ring within ring, each at its proper adjustment, to compensate for the effects of spherical aberration. The ring round that centre lens is ground so as to obviate what would otherwise give rise to spherical aberration; and the next ring being corrected in the same manner, you will perceive, if you look at the disc of light thrown by the apparatus upstairs, that there is nothing like the amount of aberration that there would have been if it had been one great bull’s-eye. Here is one of Fresnel’s lamps of the fourth order so constructed (fig. 57): observe the fine effect obtained by these different lenses, as you see them revolve before you, and understand that all this upper part is made to form part of the lens, each prism throwing its rays to increase the effect; and although you may think it is imperfect, because, if you happen to sit below or above the horizontal line, you perceive but little if any of the light, yet you must bear in mind that we want the rays to go in a straight line to the horizon. So that all that building up of rings of glass is for the purpose of producing one fine and glorious lens of a large size, to send the rays all in one direction. Here is another apparatus used to pull the rays down to a horizontal sheet of light, so that the mariner may see it as a constant and uniform fixed light. The former lamp is a revolving one, and the light is seen only at certain times, as the lenses move round, and these are the points which make them valuable in their application.