Page:Various Forces of Matter.djvu/173

Rh 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 shown (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