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prescription of a friend. Mr. Bain wrote one for me. 

 He said : " Your present knowledge must forge the links 

 of connection between what has been already achieved 

 and what is now required." 



In these words he admonished me to review the past 

 and recover from it the broken ends of former investi- 

 gations. I tried to do so. Previous to going to Switz- 

 erland I had been thinking much of light and heat, of 

 magnetism and electricity, of organic germs, atoms, 

 molecules, spontaneous generation, comets and skies. 

 With one or another of these I now sought to re-form 

 an alliance, and finally succeeded in establishing a kind 

 of cohesion between thought and light. The wish grew 

 within me to trace, and to enable you to trace, some of 

 the more occult operations of this agent. I wished, if 

 possible, to take you behind the drop-scene of the senses, 

 and to show you the hidden mechanism of optical 

 action. For I take it to be well worth the while of the 

 scientific teacher to take some pains, and even great 

 pains, to make those whom he addresses co-partners of 

 his thoughts. To clear his own mind in the first place 

 from all haze and vagueness, and then to project into 

 language which shall leave no mistake as to his mean- 

 ing which shall leave even his errors naked the defi- 

 nite ideas he has shaped. 



A great deal is, I think, possible to scientific exposi- 

 tion conducted in this way. It is possible, I believe, 

 even before an audience like the present, to uncover to 

 some extent the unseen things of nature, and thus to 

 give, not only to professed students, but to others with 

 the necessary bias, industry and capacity, an intelligent 

 interest in the operations of science. Time and labor 

 are necessary to this result, but science is the gainer 

 from the public sympathy thus created. 



