320 NINETEENTH CENTURY. _ pt. hi. 



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 concluded 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. I have 

 not been able to find so simple an instrument as this ; the 

 cheapest sold by Mr. Browning, the famous spectroscope 

 maker, in the Strand, costs twenty-two shillings, and with 

 this you may see the black lines clearly when you turn 

 it to the sun. But if this is not to be had, you may gain 

 some idea of the principle of the dark lines by the following 

 illustration. Colour a strip of paper exactly like the con- 

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

 very narrow strips and place them in order side by side on a 

 dark ground ; each strip will represent an image of the slit, 

 and the whole will be a continuous spectrum as before. But 

 now suppose one set of waves to be wanting ; take out one 

 of your strips and you will have a dark space. This repre- 



