The following figure may perhaps illustrate what has been now stated. Let LL (fig. 53.) represent a convex lens of _crown glass_, and _ll_ a concave lens of _flint glass_. A ray of the sun S, falls at F on the convex lens which will refract it exactly as the prism ABC, whose faces touch the two surfaces of the lens at the points where the ray enters and quits it. The solar ray, SF, thus refracted by the lens LL, or prism ABC, would have formed a spectrum PT on the wall, had there been no other lens, the violet ray F crossing the axis of the lens at V, and going to the upper end P of the spectrum; and the red ray FR, going to the lower end T. But as the flint-glass lens _ll_, or the prism A_a_C which receives the rays FV, FR, at the same points, is interposed, these rays will be united at _f_, and form a small circle of white light; the ray SF of the sun being now refracted without colour from its primitive direction SFY into the new direction F_f_. In like manner the corresponding ray SM will be refracted to _f_, and a white and colourless image of the sun will be there formed by the two lenses. In this combination of lenses it is obvious that the spherical aberration of the flint lens corrects to a considerable degree that of the crown-glass, and by a proper adjustment of the radii of the surfaces, it may be almost wholly removed. This error is still more completely corrected in the _triple_ achromatic object-glass, which consists of three lenses--a concave flint lens placed between convexes of crown glass. Fig. 54 shows the _double_ achromatic lens, and fig. 55, the _triple_ object-glass, as they are fitted up in their cells, and placed at the object end of the telescope. In consequence of their producing a focal image free of colour they will bear a much larger aperture and a much greater magnifying power than common refracting telescopes of the same length. While a common telescope whose object-glass is 3-1/2 feet focal distance will bear an aperture of scarcely 1 inch, the 3-1/2 feet Achromatic will bear an aperture of 3-1/4 inches, and consequently transmits 10-1/2 times the quantity of light. While the one can bear a magnifying power of only about 36 times, the other will bear a magnifying power for celestial objects of more than 200 times. [Illustration: _figure 54._] [Illustration: _figure 55._]