INDUCTION. AND DEDUCTION. 265 



clearly Tisible. I do not propose to enter bere into the improvement of 

 the optical apparatus, nor to explain liow the fugitive images of Da- 

 guerre were, by gilding in a chemical way, rendered stable and unalter- 

 able, but I return to the images on paper and will first speak of the in- 

 fluence which the Daguerrean discoveries had upon Talbot's undertaking. 



Daguerre had found that the effect of the light exerted for a second 

 on his prepared plates sufficed, through evaporization with quicksilver, 

 to bring out an image. As Talbot had on his paper the same prepara- 

 tion as Daguerre on his plates, he inferred that on the paper also, by a 

 second's illumination in the camera obscura, the sun must have pro- 

 duced an impression ; he was convinced that an image was i^resent on 

 the paper, though not a trace of it was to be seen. This conviction im- 

 pelled him to seek for some medium through which the figure might be 

 brought out ; there must be something, he thought, by which this could 

 be effected. jSTow, how came Talbot to employ a solution of gallic acid 

 for this purpose ? 



Most persons, perhaps, would be disposed to allege here, as in Da- 

 guerre's case, the iuterveution of hazard, but the choice of gallic acid 

 was no accident. Daguerre had placed the vessel containing quick- 

 silver in the press with no view to experiments ; his images were pro- 

 duced through no agency of his. Talbot, on the other hand, applied 

 himself to the research for means of accomplishing a definite object, and 

 amon^ so many thousand substances his imaginative faculty naturally 

 rejected all those which stood in no relation to that object, and dwelt 

 only on those which produce an effect similar to that of light. Now, 

 light and warm gallic acid blacken the salts of silver ; the effect of botli 

 is identical, but that of gallic acid much the stronger. The sun's light 

 had, in the camera obscura, produced an effect on the prepnred paper, 

 but one too weak to be perceptible ; perhaps it might, as he argued, be 

 brought out and strengthened by gallic acid. The trial succeeded, and 

 the correctness of the induction was thereby vindicated. 



From these examples the nature of induction ought to be intelligible 

 to every one. It will be remarked that an acquaintance with the prin- 

 ciple of the processes, how light and gallic acid properly act upon the 

 salts of silver, ichereon the solution of these salts in sub-sulphurated 

 natron depends, was for Talbot's as well as for Daguerre's purpose per- 

 fectly indifferent. 



For those persons who have no acquaintance with the ideal combina- 

 tions of the imaginative power, they naturally do not exist, and such 

 are for the most part prone to ascribe to chance discoveries which pro- 

 ceed from the most sagacious inferences of the facult3^ Chance has in- 

 deed its own part therein, siuce the elements for the determinations of 

 the understanding are so frequently offered to it by so-called accidental 

 circumstances. But the fact that experimentation must be learned ; 

 that it has its rules and is an art, and that their results presuppose a 

 very widely ranging acquaintanceshii^ with facts or sensible phenomena, 



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