104 SATUPwDAY LKCTUKES. 



the eye; the third, the eye itself; and the fourth, the ner- 

 vous center which converts the impression made on the eye 

 into a sensation. 



Of the object to be seen, but Uttle need be said. All that 

 is necessary being that it be foufid in the medium which 

 brings it in relation with the eye, that is to say, it must be 

 placed in what we call the light. As you well know, no ob- 

 ject situated in what we call darkness can be seen. The 

 object may be luminous within itself, that is, it may gener- 

 ate the light which it gives off, as in the case of the sun, 

 candle, lamp, fire, &c. ; or it may merely reflect or throw 

 off the liirht which comes from some other source, as in the 

 ease with the large majority of objects by which we are sur- 

 rounded ; but it is absolutely necessary for the purposes of 

 vision that light proceed from it. 



This light is the second link in our chain, and it is one of 

 the most wonderful and mysterious of the workings of Na- 

 ture. What is light? Is it a something, oronly an essence 

 or a spirit? What are the laws governing its action? 

 These are ciuestions which the earlier philosophers asked 

 themselves and sought, for a long time in vain, to answer. 



We will not occupy your attention with any of the 

 theories before the time of Newton, as it was his genius 

 which first placed the study of the operations of light on a 

 solid basis. 



His theory was that all luminous bodies gave off minute 

 impalpable corpuscles which passed directly into the eye 

 and affected the retina and optic nerve. This hypothesis 

 was, of course, not susceptible of demonstration, but he suc- 

 ceeded by it in accounting for many of the manifestations 

 of light in a manner more satisfactory than had been done 

 by any other theory then known. The corpuscular or 

 " emission" theory, having the weight of the great name of 

 Newton attached to it, was accepted by most philosophers of 

 his time, and, with the additional weight of tradition, af- 

 fected many w^ho came after him. All theories respecting 

 the ultimate facts of any phenomenon in nature must be 

 based largely on a priori reasoning, for no ultimate knowl- 



