Figure 61 shows the deeply curved form necessary even at half the relative aperture usable with common glasses. At _F_/20 the secondary spectrum from the latter is not conspicuous and Roe (Pop. Ast. _18_, 193), testing side by side a small Steinheil of the new glasses, and a Clark of the old, of almost identical size and focal ratio, found no difference in their practical performance. Another attack on the same problem was more successfully made by H. D. Taylor. Realizing the difficulty found with a doublet objective of even the best matched of the new glasses, he adopted the plan of getting a flint of exactly the right dispersion by averaging the dispersions of a properly selected pair of flints formed into lenses of the appropriate relative curvatures. [Illustration: FIG. 61.—Apochromatic Doublet.] [Illustration: FIG. 62.—Apochromatic Triplet.] The resulting form of objective is made, especially, by Cooke of York, and also by Continental makers, and carries the name of “photo-visual” since the exactness of corrections is carried well into the violet, so that one can see and photograph at the same focus. The residual chromatic error is very small, not above 1/8 to 1/10 the ordinary secondary spectrum. By this construction it is practicable to increase the aperture to _F_/12 or _F_/10 while still retaining moderate curvatures and reducing the residual spherical aberration. There are a round dozen triplet forms possible, of which the best, adopted by Taylor, is shown in Fig. 62. It has the duplex flint ahead—first a baryta light flint, then a borosilicate flint, and to the rear a special light crown. The two latter glasses have been under some suspicion as to permanence, but the difficulty has of late years been reported as remedied. Be that as it may, neither doublets nor triplets with reduced secondary spectrum have come into any large use for astronomical purposes. Their increased cost is considerable,[13] their aperture even in the triplet, rather small for astrophotography, and their achromatism is still lacking the perfection reached by a mirror. [13] The doublet costs about one and a half times, and the triplet more than twice the price of an ordinary achromatic of the same aperture. The matter of achromatism is further complicated by the fact that objectives are usually over-achromatized to compensate for the chromatic errors in the eyepiece, and especially in the eye. As a general rule an outstanding error in any part of an optical system can be more or less perfectly balanced by an opposite error anywhere else in the system—the particular point chosen being a matter of convenience with respect to other corrections.