Phenomena of the Rainbow. 



289 



transmission of rays after two refractions and two reflexions, 

 though they will be rendered insensible by the superior 



Fig. 7. 



quantity of light immediately refracted. But so soon as the 

 angle is altered to avoid this, an image of the luminous body is 

 perceived, which arrives by the course shown in Fig, 8. The 



Fig. 8. 



deviation goes on diminishing * till the bulb approaches an 

 angle equal to that formed by the blue edge of a secondary bow ; 

 the light then changes to the various colours of the spectrum, 

 and escapes as before in a faint red, Fig. 9. 



I have said, that in the preceding observations the image of 

 the luminous body was uncoloured, except when the bulb occu- 

 pied angular positions similar to those of the coloured parts of 

 the primary or secondary bows. It is, nevertheless, certain 

 that in all the other positions the light must have been more or 



* The expression is liable to be misunderstood. The deviation is said to be 

 nothing when the ray returns exactly upon its own path ; consequently the devia- 

 tion is a maximum, or 180, when it preserves its direction perfectly unchanged. 



