210 Experimental Philosophy. [Lecture 14 



as is evident from the experiment with the glass 

 globe, which will only refract the rays in a certain 

 position. The least refrangible, or red rays, make 

 an angle of forty-two degrees two minutes, and 

 the most refrangible, or violet rays, an angle of 

 forty degrees seventeen minutes. Now if a line 

 is drawn horizontally from the spectator's eye, it 

 is evident that angles formed with this line, of a 

 certain dimension in every direction, will produce 

 a circle, as will be evident by only attaching a 

 cord of a given length to a certain point, round 

 which it may turn as round its axis, and in every 

 point will describe an angle with the horizontal 

 line of a certain and determinate extent. 



From an analytical investigation (which, how- 

 ever, it would not be consistent with our plan to 

 introduce here* ) it results that the total breadth 

 of the interior bow is 2 15', that of the exterior 

 bow 5 40 7 , and the distance between them 8? 25'. 



We see a greater or a less part of the rainbow, 

 according as the sun is more or less elevated above 

 the horizon. When die luminary is near the 

 plane of the horizon, then the axis of vision (as 

 EF) which is at the same time, that of the cone 

 formed by all the effectual rays, coincides with 

 the horizon ; and the rainbow, in this case, is a 

 emkarcle. In proportion as the sun is elevated, 

 the axis EF sinks below its first position, and the 



It ma? be seen in a note at page 21 8, rol. ii. of Gregory's 

 translation of Hauy's Philosophy. 



