A POPULAR ACCOUNT 



N-EW 



CHAPTER I. 



Of the State of Optical Science before 

 the time of Newton. 



(1.) THE splendid phenomena of optics 

 must have been among the first natural 

 appearances to attract the attention of 

 mankind. Of all the objects in nature, 

 light is perhaps the most pleasurable. 

 Vision, at once the most perfect and 

 useful of the senses, wholly depends on 

 it. By its agency the sphere of our ob- 

 servation and experience is indefinitely 

 enlarged. It brings us sure and im- 

 mediate intelligence of existences and 

 events, whose places are remote, and 

 thus gives us a certain degree of omni- 

 presence. Setting aside all the beau- 

 tiful variety of form and figure, and the 

 gorgeous pheaomena of colours, which 

 it is the means of disclosing, light it- 

 self is a delightful perception. Nature 

 supplies it so continually and so abun- 

 dantly, that we are apt to forget its 

 value; but, in cases where habit has 

 not blunted the sense of pleasure, it 

 seems to produce singular enjoyment. 

 The infant eagerly directs its gaze to the 

 window or the lamp, and stretches forth 

 its hand as if to grasp an object so 

 agreeable. Persons blind from infancy, 

 but whose organs are not absolutely 

 opaque, derive exquisite pleasure from 

 the perception of the cloudy light which 

 the imperfectly transparent humors al- 

 low them. 



The property of light soonest noticed 

 was most probably its rectilinear pro- 

 pagation, by far the most important of 

 its qualities, and one with which all the 

 others are intimately connected. It was 

 impossible to observe the effects of 

 opaque bodies on light, the confines of 

 their shadows and their effects on the 

 sense of sight, without at once discover- 

 ing this important law. An opaque 

 body B (flg. 1.), placed in a right line 

 A C joining another opaque body A and 



aluminous point C, deprived that other 

 opaque body A of the light it received 



Fig. 1. 



from the point C ; but if the same 

 opaque body B was placed in any 

 curved or crooked line ABC, joining the 

 luminous point G and illuminated body 

 A, no such obscuration was produced. 

 Again, if a straight line be drawn from 

 the eye to a luminous point, and also 

 any curved or crooked line drawn from 

 the eye to the same point, an opaque 

 body placed any where in the straight 

 line will deprive the eye of the percep- 

 tion of light ; but if the same opaque 

 body be placed any where in the curved 

 or crooked line, the perception of light 

 continues. Facts like these must have 

 occurred so constantly at all times, and in 

 all places, the inference from them is so 

 evident and immediate, that it is impos- 

 sible to suppose that the rectilinear pro- 

 pagation of light was not known even 

 in the most rude and savage state. 



(2.) Some of the phenomena of re- 

 flection must also have attracted atten- 

 tion, and excited inquiry at a very early 

 period. The inverted picture of a land- 

 scape in the placid water of the lake or 

 river must have been viewed with as- 

 tonishment. Polished surfaces, natural 

 and artificial, presented themselves in 

 sufficient abundance to furnish nume- 

 rous experiments on reflection, and thus 

 from the rectilinear propagation of light 

 the step to the law of reflection was not 

 very difficult. The equality of the angles 

 of incidence and reflection was taught 

 in the Platonic school, and probably was 

 known considerably prior to that date. 

 B 



