158 Transactions oi the American Institute, 



SCIENTIFIC LECTURE-XI. 



ON THE SPECTEOSCOPE. 



By Pkof. J. T. Cooke, op Hakvakd College. 



The eleven til lecture of the scientific course waa delivered on 

 Wednesday evening, February 3d, 1869, at Steinway Hall. Prof. 

 Tillman, of the American Institute, presided, and introduced the 

 lecturer, who said : 



The color of light depends to a certain limited extent upon the 

 nature of the source from which it proceeds. By studying the rela- 

 tion between the two, we have reached a new method of chemical 

 analysis, by which we have been able not only to discover several 

 metallic elements among the materials of our globe, but also to extend 

 our investigations beyond the limited sphere of this planet, and to 

 reach some knowledge, however imperfect, of the constitution of the 

 heavenly bodies. To the course of reasoning through which these 

 remarkable results have been attained, I am to ask your attention 

 in my lecture this evening. All bodies when heated to a sufiicientlj 

 high temperature emit light. Indeed it can readily be shown that if 

 our theories are true, light is a necessary result of a high temperature. 

 A bar of iron heated in a blacksmith's forge, a mass of coal burning 

 in a grate, these gas-lights which illuminate our hall, are all illustra- 

 tions of this general principle. In all these cases the light emitted 

 has no peculiar color, and is what we call pure white light ; and 

 the source from which the light emanates is in eveiy case a solid body. 

 Moreover, what is true in these limited instances, we find to be a 

 universal truth, namely, that an incandescent solid always emits pure 

 white light. But before we can deduce any safe conclusion fi-om this 

 general principle, we must understand what is meant by pure white 

 light, because white light is not, as is frequently supposed, a simple 

 sensation, but, on the contrary, a very complex sensation. It is to 

 Sir Isaac Newton that we owe the first analysis of white light. 



