The eye being quite uncorrected for color the image produced even by a reflector is likely to show a colored fringe if at all bright, the more conspicuous as the relative aperture of the pupil increases. For low power eyepieces the emerging ray may quite fill a wide pupil and then the chromatic error is troublesome. Hence it has been the custom of skilled opticians, from the time of Fraunhofer, who probably started the practice, to overdo the correction of the objective just a little to balance the fault of the eye. What actually happens is shown in Fig. 63, which gives the results of achromatization as practiced by some of the world’s adepts. The shortest focus is in the yellow green, not far from the line D. The longest is in the violet, and F, instead of coinciding in focus with C as it is conventionally supposed to do, actually coincides with the deep and faint red near the line marked B. Hence the visible effect is to lengthen the focus for blue enough to make up for the tendency of the eye in the other direction. The resulting image then should show no conspicuous rim of red or blue. The actual adjustment of the color correction is almost wholly a matter of skilled judgment but Fig. 63 shows that of the great makers to be quite uniform. The smallest overcorrection is found in the Grubb objective, the largest in the Fraunhofer. The differences seem to be due mainly to individual variations of opinion as to what diameter of pupil should be taken as typical for the eye. The common practice is to get the best possible adjustment for a fairly high power, corresponding to a beam hardly 1/64 inch in diameter entering the pupil. In any case the bigger the pencil of rays utilized by the eye, i.e., the lower the power, the more overcorrection must be provided, so that telescopes intended, like comet seekers, for regular use with low powers must be designed accordingly, either as respects objective or ocular. [Illustration: FIG. 63.—Achromatization Curves by Various Makers. 1. Fraunhofer 2. Clark 3. Steinheil 4. Hastings-Brashear 5. Grubb ] The differences concerned in this chromatic correction for power are by no means negligible in observing, and an objective actually conforming to the C to F correction assumed in tables of optical glass would produce a decidedly unpleasant impression when used with various powers on bright objects. And the values for ν implied in the actual color correction are not immaterial in computing the best form for a proposed objective. 1 is from Franunhofer’s own hands, the instrument of 9.6 inches aperture and 170 inches focus in the Berlin Observatory. 2 The Clark refractor of the Lowell Observatory, 24 inches aperture and 386 inches focal length. This is of the usual Clark form, crown ahead, with lenses separated by about ⅙ of their diameter. 3 is a Steinheil refractor at Potsdam of 5.3 inches aperture, and 85 inches focus.