26o ON LIGHT. 



of colour, it must present to his eyes what we should 

 be disposed to call a hideous monotony light and 

 shade only revealing the forms of objects as in an en- 

 graving. Yet what we never knew we never miss. 

 There may, and not improbably do, exist beings in 

 other spheres, if not here on earth, whose vision is 

 sensitive to those rays of the spectrum which extend 

 far beyond the violet or its lavender prolongation, and 

 which we know at present only by their powerful 

 photographic activity, and by their agency in producing 

 that singular species of phosphorescence in certain 

 media to which Professor Stokes has given the name 

 of Fluorescence. By these properties, the solar spec- 

 trum is proved to be prolonged far beyond its visible 

 limits at its most refracted extremity; as it is by other 

 invisible rays OF HEAT, which have been traced up to 

 nearly an equal distance beyond the extreme red in 

 the opposite direction.* All, however, whether of heat 

 or chemical influence, conform each for itself, and ac- 

 cording to its own special "refractive index," to the 

 same general law of the sines, as well as to every other 

 of those singular and complicated relations of the 

 luminous rays, we shall hereafter have to describe ; 

 and both the one and the other extending into and 

 thinning out as it were in the luminous region, just 

 as we have described the spectra of the primary colours 

 into those of each other. Such, and so wondrously 

 complex a compound is a sunbeam ! 



* See my paper in the Phil. Trans. R. S. 1842, "On the Action 

 vf the Solar Rays on Vegetable Colours." 



