COLOURS OF LIGHT. 133 



When the little bit of bright rainbow, or spectrum, 

 as it is called, is examined, it is found that the beam 

 of light is bent out of its path, and lengthened in the 

 direction in which it is bent ; and the parts nearest 

 and most distant from the original direction of the 

 light, which bound the length, are the ends, and the 

 intermediate boundaries the sides. The colours lie 

 across it from side to side ; first red, at the nearest 

 end, then yellow, and then blue ; but from the red to 

 the yellow the colour passes through every imagin- 

 able shade of orange ; from yellow to blue, it passes 

 through every shade of green ; and the blue fades 

 off in brightness till it vanishes in that soft purple 

 which often tints the clouds in the evening, and 

 sometimes in the morning, and often gives the last 

 tint to the clear sky. 



Now there is most heat in the red end ; that heat 

 is greater without, or on the edge of the colour, 

 than where it is most intense ; and it diminishes as 

 the blue end is approached, so as to be barely, if at 

 all, perceptible there. Heat is the grand agent in 

 burning, the result of which is the union of the whole 

 or part of the substance burned with that part of 

 the atmospheric air which is called oxygen; and it 

 also favours the union of oxygen with substances 

 when there is merely heat but nojlame. Substances 

 which are combined with oxygen are said to be 

 oxidized ; the red end of the spectrum, which heats 

 the most, also oxidizes the most ; and that property 

 becomes less and less till the middle is arrived at, 

 and there it is not perceptible even by the nicest 

 tests. That middle is in the green, just about that 

 shade of it which we call grass-green, and seen in 

 a well-kept lawn of fine forest grasses. At the blue 

 or most distant end, there is a property the very 

 opposite of that at the red ; and, like the former, 

 it is strongest without, or at the edge of the colour, 

 and it becomes less and less, till where the green is 

 reached it is as imperceptible as that which begins 

 M 



