282- Secondary Rainbb'Jj. [Book III. 



This altitude depends on two things, the angle be- 

 tween the incident and emergent- rays, and the height 

 of the fun above the horizon ; for fmce S M is pa- 

 rallel to S N, the angle S N M is equal- to N M I, 

 but S M H, the altitude of the fun, is equal to K M I, 

 therefore the altitude of the bow W M K, which is 

 equal to the difference between W M I and K M I, 

 is equal to the difference between the angles made by 

 the incident and emergent rays and the altitude of the 

 fun. 



The angle between the incident and emergent rays 

 is different for the different colours, as was already 

 intimated ; for the red or leaft refrangible rays it is 

 equal to 42 2' j for the violet, or moft refrangible, it 

 is equal to 40" 17' -, confequently when the fun is 

 more than 42 ?! above the horizon, the red colour 

 cannot be feen -, when it is above 40 1 7', the violet 

 colour cannot be feen. 



The fecondary bow, as I have faid, is made in a 

 fimii.ir manner, but the fun's rays fuffer, in this cafe, 

 two reflections within the drop. The ray S B is de- 

 compounded at B and one part is refracted to A, 

 thence re Reeled to P, and from P reflected to W, 

 where it is refracted to M. The angle between the 

 incident and emergent rays S N M is equal as before 

 to N M I, and N M K, the height of the bow, is equal 

 to the difference between the angle made by the inci- 

 dent and emergent rays and the height of the fun. In 

 this cafe the angle S N M, for the red rays, is equal 

 to 50 7', and for the violet rays it is equal to 54 7"; 

 confequently the upper part of the fecondary bow will 

 be feen only when the fun is above 54 7' above the 

 horizon, and the lower part of the bow will be fern 

 only when the fun is 50 7' above the horizon. 



lit 



