INSTRUMENTAL. DEVELOPMENT HALE 189 



importance developed at this time was the pendulum clock, devised 

 by Huygens following Galileo's discovery of isochronism. 



Two steps taken for the purpose of overcoming chromatic aberra- 

 tion ultimately proved successful. The reflecting telescope, intro- 

 duced by Gregory and Newton, reached apertures of 4 feet in the 

 hands of Herschel and 6 feet in those of Lord Rosse. The invention 

 of the achromatic objective, followed by the production of optical 

 glass in larger and larger disks, made way for the great refractors 

 of the present day. Their high perfection, like that of the modern 

 reflector, is the result of successive advances in the art of the glass 

 maker, the metallurgist, the mechanical engineer, and the optician, 

 and the development of modern machine tools, which Lord Rosse 

 did not possess. Even if the photographic plate had then been per- 

 fected, the absence of an accurately driven equatorial mounting 

 would have rendered it useless with his 6-foot reflector. The refine- 

 ment and precision of the modern meridian circle, with its nearly 

 perfect pivots and beautifully graduated circles, is another result of 

 the improved art of the instrument maker, which is also illustrated 

 in such valuable accessories as the latest types of clocks, the record- 

 ing chronograph, and the moving wire micrometer. 



The first telescopes collected about 80 times as much light as the 

 unaided eye, and this light-gathering power has now been increased 

 to about 200,000 times that of the eye. As the quality of the atmos- 

 phere and the optical and mechanical perfection of the best modern 

 instruments are sufficiently good to permit all of this light (barring 

 losses by reflection) to be concentrated and held in a very small 

 image, the gain thus effected is enormous. But the advantages de- 

 rived from the introduction and improvement of the photographic 

 plate, and the development of many auxiliary instruments and 

 methods, are still more important. 



When Newton decomposed sunlight with a prism in 1672, he took 

 the first great step in the initiation of spectroscopy. It was not until 

 1803, however, that Wollaston, using a narrow slit instead of New- 

 ton's wider one, detected the principal dark lines in the solar spec- 

 trum, nearly 600 of which were measured by Fraunhofer in 1814. 

 Their interpretation by Stokes, who, in 1852, recognized that the 

 double D line is due to sodium vapor, which absorbs the same radia- 

 tions that it emits, and later by Kirchhoff and Bunsen, who, in 1859, 

 identified many terrestrial elements in the sun, provided the means 

 of determining the chemical composition of celestial objects. 



The study of stellar evolution, foreshadowed by Herschel and by 

 Laplace in the nebular hypothesis, was thus rendered possible in the 

 very year of the publication # of Darwin's Origin of Species. 

 This was a tremendous advance, even when only the classification 

 of stellar spectra, at once undertaken by Secchi and Huggins, and 



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