Scientific Lectures. 135 



placed in the path of the beam, brought to a focus the rays from the 

 round opening in the lantern, through which the light came. On 

 introducing a prism, the beam was bent out of its, course, and was 

 also decomposed into a brilliant rainbow band of color. An elongated 

 opening or slit was then substituted for the round one, and the beam 

 was dispersed by two carbon-disulphide prisms. A splendid spectrum, 

 four feet broad and twenty feet long, appeared upon the screen.] 

 We may, therefore, define a spectrum as the colored image obtained 

 when light passes through a prism. 



By what means, now, may spectra be best produced and examined ; 

 or, in other words, what are the essential parts of a spectroscope ? 

 We reply : there must be, first, the slit or narrow opening through 

 which the light comes, and a lens to render the rays from it parallel; 

 second, a prism for effecting the dispersion ; and third, an observing 

 telescope by which the spectrum is viewed. Every spectroscope must 

 have these three parts ; but having these, the details of arrangement 

 may be widely different. The first spectroscope used by Bunsen con- 

 sisted of a quadrangular box containing the prism, and carrying the 

 telescopes in its sides. For purposes of solar research, spectroscopes 

 with several prisms, like those of Kirehhoff, Lockyer and Young, 

 are employed, in order to increase the length of the spectrum. [By 

 means of the lime-light, pictures of Duboscq's single-prism spectro- 

 scope — one of the most convenient forms fur chemical work — of 

 KirchhofPs four-prism spectroscope, and of Lockyer's and Young's 

 telespectroscopes, were projected on the screen and described.] 



To what problems, lastly, may the spectroscope be applied ; and 

 what may be learned from the study of spectra ? It is evident at the 

 outset that the spectrum of white light, in which all the colors are 

 found, must differ from the spectrum of colored light, in which, by 

 its very nature, some colors are or may be absent. White light gives 

 what is called a continuous spectrum, because the seven colors shade 

 inperceptibly into each other without any abrupt transition. Colored 

 light, on the other hand, gives a discontinuous or interrupted spectrum, 

 in which some of the colors are wholly or partially wanting. If, now, 

 we remember that whenever a solid or liquid mass is raised to incan- 

 descence, the light emitted by it is always finally white, and that 

 when a gaseous body is thus heated the light which it emits is colored, 

 it is clear that we may determine by the spectroscope whether the 

 source of light we are considering is in the solid or gaseous condi- 

 tion. Moreover, since all continuous spectra are alike, the spectro- 

 scope can tell us nothing about the character of an incandescent solid 



