Manchester Memoirs, Vol. xliv. (1900), No. I*i. 5 



As I had only a single grating, however, with whkh to 

 make my experiments, it was necessary therefore to adopt 

 some other means, both of illumination and of super- 

 position, than those adopted by Professor Wood. 



Three sources of light became manifestly necessary, 

 and, to show the pictures stereoscopically, six. 



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