C28 TELESCOPE ing their origin in the want of a perfect cor- relation of the colored spaces in the spectra formed by the crown and flint lens; so that if any two colors be made to unite at the same focus, as in ordinary object glasses, there would not be at the same time a complete union of the rest. This want of correlation is called the "irrationality" of the colored spaces, and its effect is called the "secondary spectrum." Dr. Blair, to overcome this effect, first made each of the lenses of his object glass indepen- dently achromatic, and in such a way that their secondary spectra corrected each other. This he accomplished by using fluid media, two lenses of which were enclosed in combination with three of glass. Moreover, in the course of his experiments, he discovered that muri- atic acid combined in proper proportions with metallic antimony gave a spectrum in which the colors had exactly the same proportions as in crown glass; and therefore by enclosing this fluid between two crown lenses, one a plano-convex and the other a meniscus, he ob- tained a telescope absolutely free from colors. The name "aplanatic," or without error, was given to this combination. Another fluid-lens telescope, of the dialytic form, was constructed by Barlow, who made use of the high disper- sive power of sulphuret of carbon, a beauti- fully transparent and colorless fluid. He was able to render achromatic a combination of a crown lens 8 in. in diameter with a fluid lens of half the size. There is however a practi- cal objection to the use of sulphuret of carbon arising from the variability of its density by variations of temperature. K everting to what may be called the regular construction of achro- matics, we find that the successors of Fraun- hofer at Munich, and Guinand and Cauchoix at Paris, have produced object glasses of di- mensions far superior to those of the Dor pat lens. Disks of 10, 12, and even more inches in diameter have become familiar to these master opticians, whose skill in working them keeps even pace with their manufacture ; and in three Munich telescopes, two with more than 15 in. of clear aperture, one at Pulkova, another at the observatory of Harvard col- lege, and the third at Greenwich (aperture 13 in.), have been in use for years. The two for- mer have been the means of adding largely to the stores of astronomical knowledge ; the Greenwich telescope has not been much used. The few attempts made in the United States to manufacture optical flint glass have hither- to been but partially successful, and that with only small disks; but the American-wrought object glasses have earned for themselves a high place. Many have been made in New York by Henry Fitz, whose largest glass, 13 in. in diameter, was made for the Dudley ob- servatory at Albany. Spencer, famous for the excellence of his microscopic objectives, made for Hamilton college a 13^-inch telescope, which is highly commended. But in exqui- siteness of workmanship and performance, the object glasses made by Alvan Clark of Cam- bridge, Mass., have fairly distanced all competi- tors, native or foreign. Whoever will glance over the list of close double stars discovered with his 7- and 8-inch lenses (see " American Journal of Science," vols. xxv. and xxix.) will remark several stars that must have passed unnoticed under the review of Struve with his superior optical power. (See CLARK, ALVAN.) Mr. Dawes, one of the most skilful astronomi- cal observers of his day, took in succession five or six large refractors from Clark (disposing of each in favor of a successor including some improvement of construction which had sug- gested itself), and these, scattered throughout England, attested the skill of the American optician in the special work of figuring object glasses, in which at present he and his sons are unrivalled. In 1859 Clark began the con- struction of a magnificent object glass of 18 J in. clear aperture and with a focal distance of 23 ft., at that time the largest in the world. It was made from disks of Birmingham glass, which have a uniform density and freedom from veins, and, though only rudely mounted at first, quickly revealed the duplicity of the minute companion of a? Capricorni. In Janu- ary, 1862, it detected a companion to Sirius, perhaps the hitherto invisible one whose work- ings have been indirectly manifested in the variable movement of the larger star. This masterpiece, prevented from reaching its ori- ginal destination, was secured for the Chicago observatory. In 1870 Clark was authorized by congress to begin the construction of a telescope 24 in. in aperture for the Washington observatory ; but before the work was entered upon, the proposed aperture was changed to 26 in., Mr. Kewall of Gateshead, England, having had a glass constructed for him by Cooke and sons, York, of the hitherto unequalled aperture of 25 in. The disks of glass, obtained by Clark from Chance and co. of Birmingham, reached Cambridge, Mass., in December, 1871, and the grinding was begun in January, 1872. " Owing to the great size of the glasses," says Prof. Newcomb, "the first rough grinding was done by machinery, the 'grindstone' being a rapidly revolving iron wheel, over which a stream of water and sand was kept running. The glasses were thus roughly brought to the desired shape in a few days. The forms chosen were much more simple than those usually employed in large glasses, the crown glass be- ing double convex, with an equal curvature on each face ; the flint nearly plane ,on one side, while the other was concave, with the same curvature as the crown glass. ... In the month of June, 1872, the glass was in such good shape that only an expert could see any defect whatever. Looking through it we could read, at the distance of some 400 ft., a microscopic photograph illegible to the naked eye. . . . Artificial double stars, one third of a second apart, were clearly separated. In hands less severely critical than those of the
