COLOURS. 78 



LESSON 34. 



Colours. 



Sem'icircle, a half round, part of a circle divided by the diameter, 

 Juncture, the line at which two things are joined together. 

 Prism, a solid piece of glass with three flat sides, and two equal 

 and parallel triangular ends. 



Sir Isaac Newton, to whom we are indebted for the most 

 important discoveries respecting light and colours, was the 

 first who divided a white ray of light, and found it to con- 

 sist of an assemblage of coloured rays. This separation 

 may be observed in the well known experiment of the prism. 

 A ray being let into a darkened room, through a small round 

 aperture in the shutter, and falling on a triangular glass 

 prism, is, by the refraction of the prism, considerably dilat- 

 ed, and it will exhibit, on a skreen or on the opposite wall, 

 an oblong image called a spectrum, variously coloured ; the 

 extremities of which are bounded by semicircles, and the 

 sides are rectilinear. The colours are commonly divided 

 into seven, which, however, have various shades gradually 

 intermixing at their juncture. The following lines from 

 Blackmore represent their order, beginning at the side of the 

 refracting angle of the prism. 



Of parent colours, first the flaming red 

 Sprung vivid forth ; the tawny orange, next ; 

 And next, delicious yellow ; by whose side 

 Fell the kind beams of all-refreshing green; 

 Then the pure blue, that swells autumnal skies, 

 Ethereal played ; and then, of sadder hue, 

 Emerged tfie deepened indigo, as when 

 The heavy skirted evening droops with frost, 

 Whilethe last gleamings of refracted light 

 Died in the fainting violet away. 

 The union of these colours, in the proportions in which 

 they appear in the spectrum, produce in us the idea of 

 whiteness. If you paint a card in compartments with these 

 seven colours, and whirl it rapidly on a pin, it will appear 

 white. But a more decided proof of the composition of a 

 white ray is afforded by uniting these coloured rays, and 

 forming with them a ray of white light. This can be done 

 7 



