CH. xxxiv. FRAUNHOFER'S DISCOVERIES. 335 



From having been constantly at work as an optician, 

 Fraunhofer had been led to study the subject of light, and 

 among other experiments he repeated those of Newton ; and 

 it happened that he too used a narrow slit, as Wollaston 

 had done. Thus he also noticed the black lines which 

 divided the colours, and by making his slit very narrow and 

 using prisms of very pure glass he discovered in a ray of 

 sunlight no less than 576 of these black lines. Plate I., 

 No. 2, gives a few of the principal of these, to which he put 

 letters, and which have ever since been called ' Fraunhofer's 

 lines.' As none of these lines appear when the light of a 

 candle or lamp is passed through a prism, Fraunhofer con- 

 cluded that sunlight must be defective, and some of its 

 coloured rays must be missing. For, as numberless waves 

 of coloured light are passing through the slit and the prism 

 spreads them out so that each set of waves makes an 

 upright image of the slit on the spectrum, if any waves were 

 missing there would be a dark image of the slit instead of 

 a coloured one. 



By far the best way of understanding this is to see it for 

 yourself. Sir John Herschel says that a little inexpensive 

 instrument may be easily made with a hollow tube of metal, 

 blackened inside, a prism fixed in it, and a metal plate with 

 a narrow slit fastened across the end of the tube, and Mr. 

 Knobel tells me he has made one still more simply by 

 placing a prism in a slit at one end of a cigar-box, and 

 looking through another slit at the other end. With 

 this several lines may be seen, but if even this is 

 not to be had, you may gain some idea of the prin- 

 ciple of the dark lines by the following illustration. 

 Colour a strip of paper exactly like the continuous 

 spectrum, No. i, Plate I., and then cut it across into very 

 narrow strips and place them in order side by side on 



