34 



SCIENCE. 



[N. S. Vol. II. No. 28. 



limb, if the central regions of the disk have 

 been rightly timed. 



Kirehhofif in 1 85S formulated the following 

 principles of spectrnm analj'sis: (1) Solid 

 and liquid bodies (also gases under high 

 pressure) give, when incandescent, a con- 

 tinuous spectrum; (2) gases under low pres- 

 sure give a discontinuous but characteristic 

 bright-line spectrum ; (3) when white light 

 passes through a gas, this medium absorbs 

 rays of identical wave-length with those com- 

 posing its own bright-line spectrum. These 

 principles fully account for the discoutin- 

 iious spectrum of the Sun, crossed, as it is, by 

 the multitude of Fraunhofer lines. But it 

 must be observed that the relative position 

 of these lines will vary with the nature of 

 the spectroscope used ; with a prism spectro- 

 scope the relative dispersion in different 

 parts of the spectrum varies with the ma- 

 terial of the prism ; with a grating spectro- 

 scope (in which the dispersion is produced 

 bj^ reflection from a gitter or grating, ruled 

 upon polished speculum metal with many 

 thousand lines to the inch), the dispersion 

 is wholly independent of the material of the 

 gitters, and it is called, therefore, the nor- 

 mal solar spectrum. Compared with this 

 a prismatic spectrum has the red end un- 

 duly compressed, and the violet end as un- 

 duly expanded. 



Rutherfurd, assisted by Chapman, ruled 

 excellent gratings mechanically ; but the 

 last degree of success has been attained by 

 Professor Rowland, of Baltimore, whose 

 ruling engine covers specular surfaces, 

 either plane or concave, six inches in diam- 

 eter with accurate lines, up to 20,000 to the 

 inch. The concavity of tlie gratings vastly 

 simplifies the accessories of the spectro- 

 scope for researches in which they are ap- 

 plicable. So great is the dispersion obtain- 

 able that the solar spectrum, as photo- 

 graphed by Rowland mth one of these 

 gratings and enlarged three-fold, is about 

 forty feet in length. The superiority of his 



ruling engine consists primarily in the ac- 

 curate construction and j)erfect mou.nting of 

 the screw, which has 20 threads to the inch, 

 and is a solid cylinder of steel, about 15 

 inches long and 1^ inches in diameter. 

 (Article ' Screw,' Encydopczdia Britannica, 

 9th edition.) The perfect gratings ruled 

 with this engine are now supplied to phj'si- 

 cists all over the world. 



By means of a spectroscope properly 

 arranged with suitable accessories, the 

 Sun's spectrum has been both delineated 

 and photographed alongside of the spectra 

 of numerous terrestrial substances. Fore- 

 most among recent investigators in this 

 field, and in mapping the solar spectrum, 

 are Thollon in France, Lockj'er and Higgs 

 in England, Thalen in Sweden, Smyth in 

 Scotland, and in America Rowland, Young, 

 Trowbridge and Hutchins. Their research, 

 together with that of previous investigators, 

 principalljr Kirchhoff and Angstrom, Vogel 

 and Fievez, has led to the certain detection 

 of at least 35 elemental substances in the 

 Sun, among which are : 



(Al) Aluminium. (Ag) Silver. 



(Ba) Barium. (C) Carbon. 



(Cd) Cadmium. (Ca) Calcium. 



(Co) Cobalt. (Cu) Copper. 



(Cr) Chromium. (Fe) Iron. 



(H) Hydrogen. (Mg) Magnesium. 



(Mn) Manganese. (Ni) Nickel. 



(Na) Sodium. (Si) Silicon. 



(Sc) Scandium. (Ti) Titanium. 



(V) Vanadium. (Zn) Zinc. 



Hydrogen, iron, nickel, titanium, calcium 

 and manganese are the most strongly mark- 

 ed. All the oxygen lines of the solar spec- 

 trum are due to the oxj'gen of our atmos- 

 phere. Chlorine and nitrogen, so abundant 

 on the Earth, and gold, mercury, phosphorus 

 and sulphur, are as yet undiscovered. Also 

 the solar spectrum appears to indicate the 

 existence of many metals in the Sun not now 

 recognized upon the Earth ; but it must be 

 remembered that our globe is known only 



