OF NEWTON'S OPTICS. 



II 



to give the reader, in a popular form, an 

 account of Newton's Optics, a work 

 which communicated to the world a 

 series of discoveries of transcendent 

 beauty, and which would themselves 

 have been sufficient to have conferred 

 immortality on the name of Newton, 

 even had he never written his PRIN- 



C1PIA. 



CHAPTER II. 



The Compound Nature of Solar Light 

 established by Experiments. 



(16.) BEFORE he enters upon the in- 

 teresting detail of the experiments which 

 led to his great optical discoveries, New- 

 ton explains in eight axioms the leading 

 principles and established results of the 

 science before his time. We have, in 

 the preceding chapter, enlarged more 

 fully on the subject than he has done in 

 the axioms, that the reader may be the 

 better enabled to appreciate the dis- 

 coveries we have to explain. The 

 propositions respecting the properties 

 of light which Newton gives under the 

 title of axioms, are not to be understood 

 as partaking of the nature of mathe- 

 matical axioms. On the contrary, many 

 of them are results, not only of careful 

 experiment, but of very subtle and com- 

 plex reasoning. " I have," says New- 

 ton, " now given, in axioms and their 

 explications, the sum of what hath been 

 hitherto treated of in optics. For what 

 hath been generally agreed on, I con- 

 tent myself to assume, under the notion 

 of principles, in order to what I have 

 further to write. And this may suffice 

 for an introduction to readers of quick 

 wit and good understanding, not yet 

 versed in optics ; although those who 

 are already acquainted with this science, 

 and have handled glasses, will more 

 readily apprehend what followeth." 



The following are the propositions 

 assumed by Newton as axioms : 



Axiom I. 



The angles of reflection and refraction 

 lie in one and the same plane with the 

 angle of incidence. 



Axiom II. 



The angle of reflection is equal to the 

 angle of incidence. 



Axiom III. 



If the refracted ray be turned directly 

 back to the point of incidence, it shall be 

 refracted into the line before .described 

 by the incident ray. 



Axiom IV. 



Refraction out of the rarer medium 

 into the denser, is made towards the 

 perpendicular ; that is, so that the 

 angle of refraction be less than the 

 angle of incidence. 



Axiom V. 



The sine of incidence is either accu- 

 rately or very nearly in a given ratio to 

 the sine of refraction. 



Axiom VI. 



Homogeneal rays which flow from 

 several points of any object, and fall 

 perpendicularly, or almost perpendicu- 

 larly, on any reflecting or refracting 

 plane or spherical surface, shall after- 

 wards diverge from so many other 

 points, or ,be parallel to so many other 

 lines, or converge to so many other 

 points, either accurately or without any 

 sensible error ; and the same thing will 

 happen, if the rays be reflected or re- 

 fracted successively by two or three or 

 more plane or spherical surfaces. 



Axiom VII. 



Wherever the rays, which come from 

 all the points of any object, meet again 

 in so many points, after they have been 

 made to converge by reflection or re- 

 fraction, there they will make a picture 

 of the object upon any white body on 

 which they fall. 



Axiom VIII. 1 



An object seen by reflection or re- 

 fraction, appears in that place from 

 whence the rays, after their last reflec- 

 tion or refraction, diverge in falling on 

 the spectator's eye. 



The work opens with the announce- 

 ment of the discovery that LIGHTS 



WHICH DIFFER IN COLOUR, DIFFER 

 ALSO IN REFRANGIBILITY. 



For those readers who are not familiar 

 with the science, we shall first explain 

 the meaning of this property, and next 

 describe the nature of the experiments 

 by which Newton established it. 



Let S &ss,jig. 5, be a section of any 

 transparent medium, as water or glass. 

 Let R be a ray of red light incident at 

 I, and supposed to proceed from some 

 red object in the direction I R, or to 

 have been transmitted through red 

 glass. In like manner, let B be a ray 

 of blue light incident at I', in a direction 

 B I', parallel to R I, and proceeding as 

 before from a blue object in that direc- 

 tion or transmitted through blue glass. 

 The angles of incidence . formed by the 



