CHAPTEE XL 



THE NEW METHOD APPLIED TO LABORATORY WORK. 



ATTENTION must next be drawn to another method of obser- 

 vation of spectra, or rather to the extension of the method we 

 have seen applied to the localization of solar phenomena, to a 

 different line of work. 



Kirchhoff, as we have seen, examined the sun as a whole, 

 and compared it with the light of a light-source as a whole. 

 We have stated the difference in the results obtained when 

 we pass from the method of observing the sun as a whole to 

 that other more detailed one of observing a small portion at a 

 time. 



Now is it worth while to do this with the light-source ? that 

 is the question. Let us deal with some simple considerations 

 which should enable us to give an answer to this question. 



The spectroscope, however simple or complex it may be, is an 

 instrument which allows us to observe the image of the slit 

 through which the light enters it, in the most perfect manner. 

 If the light contains rays of every wave-length, then the images 

 formed by each will be so close together that the spectrum will 

 be continuous, that is, without break. If the light contains 

 only certain wave-lengths, then we get certain, and not all, of 

 the possible images of the slit, and the spectrum will be 

 discontinuous. 



Again, if we have an extremely complex light-source, let us 



