COLOURED FRINGES. 



receive the violet ray which enters in the direction L v as if suchs 

 ray had proceeded from v, and it will receive the red ray which* 

 enters it in the direction n as if it had proceeded from R' ; the red 

 will therefore appear at the top, and the violet at the bottom of 

 the spectrum, when the refracting angle B of the prism is turned 

 downwards. 



But if the refracting angle B be turned upwards, as repre- 

 sented in fig. 14, then the red will appear at the bottom, and 

 the violet at the top of the spectrum, as will be perceived from 

 the figure. 



Fig 14. 



27. In general, when objects are viewed through a prism they 

 appear with their proper colours, except at their boundaries, 

 where they are fringed with the prismatic tints in directions- 

 parallel to the edge of the refracting angle of the prism. 



Let A A M M, (fig. 15,) be a small rectangular object seen upon a 

 black ground, the sides A M being vertical, and A A and M M 

 horizontal. Let us first suppose that this object has the colour 

 of a pure homogeneous red. If this object be viewed through a 

 prism whose refracting angle is directed upwards with its edge- 

 horizontal, it will be seen in a more elevated position, such as- 

 a a m m, as already explained. 



Let us next suppose that the object A A M M has the colour 

 of a pure homogeneous orange. When viewed through the prism 

 it will, as already explained, appear in a position b b tin, a. little 

 above a a m m. 



If we next suppose the object A A M M to be coloured with 

 homogeneous yellow, it will be raised by the prism to c c o o, a, 

 little above the orange image. 



If it be next supposed to have the colour of a prismatic 

 green, it will be seen at d d p p, a little above the yellow 

 image ; and if it be coloured light blue, its image will be seen 

 at e e q q, above the green image; if it be dark blue or indigo,. 



75 



