THE BOOK OF THE LANTERN. 187 



position with that marked D in the spectrum-slide. To 

 produce such a slide it is only necessary to paste over a 

 piece of glass a piece of stout black paper, and to cut 

 out with a sharp knife, when the paper is dry, the line 

 required. A little varnish colour over the cut-out place will 

 complete the slide. In using this " effect " the audience 

 should have explained to them the theory which seeks to 

 explain the reversal of the lines in the spectrum, and at 

 the right moment the spectrum-slide is so far darkened 

 by moving the lantern-dissolver, that the clear sodium 

 line shines out brightly over the spot occupied before by 

 the dark D line. I need hardly say that the two slides 

 must be in perfect register, or the effect will be spoiled. 

 The spectrum-slide can now be once more exhibited, and 

 another bright line example placed in the other lantern 

 ready to be made visible as the sodium one was just 

 now. The spectra of all the different metals can thus be 

 illustrated by the bright lines which they afford. The 

 method may perhaps be considered rough, but the effect is 

 startling, and few among a general audience are able at 

 once to realise how it is done. 



Double refraction can be shown on the screen in the 

 following manner : A card with a simple perforation about 

 one-eighth of an inch in diameter is inserted on the lantern- 

 stage, and its image is focussed on the screen. A crystal 

 of Iceland spa is then placed between this card and the 

 objective lens, and two spots of light will become apparent 

 upon the sheet. 



It may be mentioned here that in all experiments where 

 colour is required it is better, if possible, to use coloured 



