196 LECTURE ON THE EESULTS OF SPECTKUM ANALYSIS 



"We are indebted to Newton for tiie knowledge that the beautiful tints of the 

 rainbow are the common and necessary ingredients of ordinary light. He found 

 that when white light is made to pass through a prism of glass it is decomposed 

 into the beautiful colors which are seen in the rainbow. These colors when in 

 this way separated from each other form the spectrum of the light. Let this 

 white plate represent the transverse section of a beam of white light travelling 

 towards you. Let now a prism be interposed in its path. The beam of white 

 light is not turned aside as a whole, but the colored lights composing it are de- 

 flected differently, each in proportion to the rapidity of its vibrations. An 

 obvious consequence will be, that on emerging from the prism the colored lights 

 which formed the white light will separate from each other, and in place of the 

 white light which entered the prism w^e shall have its spectritm — that is, the 

 colored lights tchich comfoscd it, in a state of separation from each other. 

 Wollaston and Fraunhofer discovered that when the light of the sun is decom- 

 posed by a prism, the rainbow colors which form its spectrum are not continuous, 

 but are interrupted by a large number of dark lines. These lines of darkness 

 are the symbols which indicate the chemical constitution of the sun. It was 

 not until recently, in the year 1859, that Kirchhofif taught us the true nature of 

 these lines. He himself immediately applied his method of interpretation to the 

 dark lines of the solar spectrum, and was rewarded by the discovery that several 

 of the chemical elements which exist upon the earth are present in the solar 

 atmosphere. 



It is my intention to bring before you this evening the results of the extension 

 of this method of analysis to the heavenly bodies other than the sun. These 

 researches have been carried on in my observatory during the last four years. 

 In respect of a large part of these investigations, viz., those of the moon, the 

 planets, and lixed stars, I have had the great pleasure of working conjointly 

 with the very distinguished chemist and philosopher. Dr. Wm. A. Miller. Half 

 a century ago Fraunhofer recognized several of the solar lines in the light of the 

 moon, Venus, and Mars, and also in the spectra of several stars. Recently, 

 Donati, Janssen, Secchi, Rutherford, and the Astronomer Royal have observed 

 lines in the spectra of some stars. Before I describe the results of our observa- 

 tions, I will state, in a few words, the principles of spectrum analysis upon which 

 our interpretation of the phenomena we have observed has been based, and also 

 the method of observing which we have employed. 



"When light which has efnanated from different sources is decomposed by a 

 prism, the spectra which are obtained may differ in several important respects 

 from each other. All the spectra which may present themselves can be conve- 

 niently arranged in three general groups. A spectrum illustrating each of these 

 three orders is placed upon the diagram : 



1. The special character which distinguishes spectra of the^r*^; order consists 

 in that the continuity of the colored band is unbroken either by dark or bright 

 lines. By means of the electric lamp, Mr. Ladd will throw a spectrum of this 

 order upon the screen. We learn from such a spectium that the light has been 

 emitted by an opaque body, and almost certainly by matter in the solid or liquid 

 state. A spectrum of this order gives to us no knowledge of the chemical nature 

 of the incandescent body from which light comes. In the present case the light 

 is emitted by the white-hot carbon points of the electric lamp. A spectrum in all 

 respects similar would be formed by the light from incandescent iron, or lime, 

 or magnesia. 



2. Spectra of the second order are very different. These consist of colored 

 lines of light separated from each other. From such a spectrum we may learn 

 much. It informs us that the luminous matter from which the light has come is 

 in the state of gas. It is only when a luminous body is free from the molecular 

 trammels of solidity and liquidity that it can exhibit its own peculiar power of 

 radiating some colored rays alone. Hence substances, when in a state of gas, 



