58 HISTORY OF OPTICS. 



cal instruments, diacaustic curves, (that is, tlie curves of intense light 

 produced by refraction,) and to various other cases ; and was, of course, 

 tested and confirmed by such applications. It Avas, however, impossi- 

 ble to pursue these applications far, without a due knowledge of the 

 laws by which, in such cases, colors are produced. To these we now 

 proceed. 



[2nd Eel.] [I have omitted many interesting parts of the history of 

 Optics about this period, because I was concerned with the inductive 

 discovery of laws, rather than with mathematical deductions from such 

 laws when established, or applications of them in the form of instru- 

 ments. I might otherwise have noticed the discovery of Spectacle 

 Glasses, of the Telescope, of the Microscope, of the Camera Obscura, 

 and the mathematical explanation of these and other phenomena, as 

 given by Kepler and others. I might also have noticed the progress 

 of knowledge respecting the Eye and Vision. We have seen that 

 Alhazen described the structure of the eye. The operation of the 

 parts was gradually made out. Baptista Porta compares the eye to 

 his Camera Obscura (Magia, JVaturalis, 1579). Schemer, in his Ocu- 

 lus, published 1652, completed the Theory of the Eye. And Kepler 

 discussed some of the questions even now often agitated ; as the causes 

 and conditions of our seeing objects single with two eyes, and erect 

 with inverted images.] 



CHAPTER III. 

 DISCOVERY OF THE LAW OF DISPERSION BY REFRACTION 



EARLY attempts tvere made to account for the colors of the rain 

 bow, and various other phenomena in which colors are seen to 

 arise from transient and unsubstantial combinations of media. Thus 

 Aristotle explains the colors of the rainbow by supposing 1 that it is 

 I'ight seen through a dark medium : " Now," says he, " the bright seen 

 through the dark appears red, as, for instance, the fire of green wood 

 seen through the smoke, and the sun through mist. Also 2 the weaker 

 is the light, or the visual power, and the nearer the color approaches 

 to the black ; becoming first red, then green, then purple. But 3 the 



1 Meteor, iii. 3, p. 373. * Ib. p. 374. a Ib. p. 375 



