480 PROCEEDINGS OF THE AMERICAN ACADEMY 



dictator to planetary and sidereal systems. But these triumphs, 

 complete in their details, and grand iu tlieir cosmioal range, were 

 limited to questions which concern the distances, motions, dimen- 

 sions, and masses of the heavenly bodies. The law of gravitation can 

 assign a value to the quantity of matter in planets and binary stars; 

 but it asks and can answer no question in regard to the quality of 

 this matter, only so far as a comparison of the size and mass of a 

 body gives a measure of its density. That an instrument would be 

 invented or developed which would complement the mechanics of the 

 heavens by the chemistry of planets, comets, and stars, so that a 

 physical observatory would become a necessary adjunct of the old 

 observatory, was beyond the hope of the most sanguine astronomer, 

 down to the moment of its actual realization. 



Newton owes his singular fame, not exclusively to his discovery 

 and expansion of the law of gravitation, but partly to his experimental 

 researches in optics. That he did not recognize the dark lines in the 

 solar spectrum has been explained by the statement that he was 

 obliged to use the eye of an assistant in these experiments, on account 

 of an injury to his own. Be this as it may, the existence of these 

 lines was first known to Wollaston in 1802 ; and from that moment 

 the spectroscope and spectrum-analysis, as we now understand them, 

 were possibilities. 



Although Fraunhofer made a careful study of these lines in 1824, 

 and Brewster, Herschel, Talbot, Draper, and many others, pursued 

 the inquiry by way of experiment and explanation, and stood upon 

 the threshold of a great discovery, the spectroscope and spectrum- 

 analysis, as practical realities, date from the investigations of Kirch- 

 hoff and Bunsen, in 1862. Not only does the spectroscope carry 

 chemistry into regions tenanted only by planets, comets, stars, and 

 nebulae, and reveal motions in the direction of the line of vision 

 otherwise hopelessly beyond recognition, but it competes with the 

 ordinary chemical analysis of bodies which can be handled, and has 

 detected new substances which had escaped the vigilance of the chem- 

 ist. Some of these results can be realized with simple instruments : 

 others require a compound spectroscope consisting of a battery of 

 prisms. It was a great step in the way of simplicity and ease of 

 manipulation, when the diffraction-spectrum, produced by fine lines 

 ruled upon glass or metal, was substituted for the spectrum produced 

 by the combined refractions of many prisms. And here we touch 

 upon the researches of Professor Rowland in light, which enhance his 

 claim to the Kumford premium. 



