46 THE PROGRESS AND SPIRIT OF 



the percipient being.* This, in truth, is one of the cases now 

 frequent in science, where even failure may become a fountain 

 of discovery. Negative conclusions have their value as well 

 as positive ones, and often form an index to the path through 

 which truth and certainty are finally reached. 



Without going into details, we may allude here to the 

 recent photo-chemical researches of Bunsen and Roscoe ; to 

 those of Becquerel on phosphorescent and fluorescent bodies ; 

 and, still more, to the remarkable experiments of St. Victor 

 and Grove ; all showing the direct action of light upon the 

 molecules of matter to be far more universal and minute, as 

 well as more definite and lasting, than was before dreamt of 

 in our philosophy. In man, it has been the general pre- 

 sumption that light finds ingress by the eye alone, and with 

 the sole effect of giving instant vision of things without. 

 But recent enquiry calls upon us to recognise, even in the 

 eye itself, the retention and probably reflex actions of light 

 within the precincts of the organ ; affording, through what we 

 may venture to call photographic impressions on the retina, 

 the only plausible explanation of its subjective functions, and 

 of other phenomena little heeded from their familiarity, but 

 presenting problems of the highest interest to philosophy. 

 The experiments of D'Arcy prove that the impression of light 

 is often retained on the retina for fully two and a half minutes, 

 the time in which a luminous particle or undulation passes 

 through nearly thirty millions of miles of space ! What is the 

 condition of Light be it conceived as matter, or motion, 

 or force when thus arrested and enchained in a living orga- 

 nisation ? In this brief question lies one of the most profound 



* The curious phenomena of colour-blindness afford illustration of this 

 remark. This subject is treated of by Sir J. Herschel in a recent Memoir ; in 

 which also, with his wonted ability, he revises generally the mutual relations 

 of the prismatic colours, and presses upon the notice of the experimentalist 

 the important distinction between the study of pigments or negative colours, 

 and prismatic or positive ones. 



