184 DISSERTATION SECOND. [fart i. 



3. From Kepler to the commencement of New- 

 ton's Optical Discoveries. 



The rainbow had, from the earliest times, been an ob- 

 ject of interest with those who bestowed attention on op- 

 tical appearances, but it is much too complicated a phe- 

 nomenon to be easily explained. In general, however, it 

 was understood to arise from light reflected by the drops 

 of rain falling from a cloud opposite to the sun. The diffi- 

 culty seemed to be how to account for the colour, which 

 is never produced in white light, such as that of the sun, 

 by mere reflection. Maurolycus advanced a considerable 

 step when he supposed that the light enters the drop, and 

 acquires colour by refraction ; but in tracing the course of 

 the ray he was quite bewildered. Others supposed the 

 refraction and the colour to be the effect of one drop, and 

 the reflection of another ; so that two refractions and one 

 reflection were employed, but in such a manner as to be 

 still very remote from the truth. 



Antonio de Dominis, Archbishop of Spalatro, had the 

 good fortune to fall upon the true explanation- Having 

 placed a bottle of water opposite to the sun, and a little 

 above his eye, he saw a beam of light issue from the under 

 side of the bottle, which acquired different colours, in the 

 same order, and with the same brilliancy as in the rain- 

 bow, when the bottle was a little raised or depressed. 

 From comparing all the circumstances, he perceived that 

 the rays had entered the bottle, and that, after two re- 

 fractions from the convex part, and a reflection from the 

 concave, they were returned to the eye tinged with differ- 

 ent colours, according to the angle at which the ray had 

 enlered. The rays that gave the same colour made the 

 same angle with the surface, and hence all the drops that 



