LIGHT. 31 



medium to another, differs in refrangibility. A ray of 

 light is found to contain seven colours, red, orange, yellow, 

 green, blue, indigo, and violet: these are called primary 

 colours, but these seven are reduceable to three, red, 

 blue, and yellow. If a hole be made in a shutter in a 

 darkened room, and a ray of light be made to pass 

 through a prism held obliquely, and received on a screen 

 at a proper distance, the ray will be found to be divided 

 into the seven primary colours just named ; and if the 

 image of the rays (called a spectrum) be divided into 

 360 equal parts, the red will be found to occupy 45 of 

 these parts, the orange 27, the yellow 4S, the green 60, 

 the blue 60, the indigo 40, and the violet 80. The rain- 

 bow, which is formed by the refraction and reflection of 

 the rays of the sun in drops of falling rain, most beauti- 

 fully exhibits the natural separation of the colours of 

 Light. 



Of the seven primary colours, the red is the least re- 

 frangible, and the violet the most refrangible. The 

 strength of the red rays is visible on a hazy morning ; 

 the other rays being unable to penetrate the mist, the sun 

 appears of a deep crimson colour. When all the rays 

 are equally refrangible, light is said to be homogeneous ; 

 and when some rays are more refrangible than others, it 

 is said to be heterogeneous. The colours of homogeneous 

 light are not altered by refraction, and anything viewed 

 in homogeneous light will appear of the colour of the 

 rays which fall upon it. Red lead or yellow ochre, for 

 instance, in a homogeneous green light will appear green, 

 and so would any other substance. 



The colours of bodies are said to arise from their dis- 

 position to reflect one sort of rays and to absorb the 

 others j thus a red substance absorbs all the rays but the 

 red, which it reflects. The whiteness of bodies arises from 

 their reflecting all the rays of light promiscuously, and 

 the blackness of bodies from their absorbing all the rays 

 of light thrown on them. That white is a compound of 

 all the primary colours, is prettily shown by painting on 





R. Phillips, different colours are different perceptible effects on the 

 optic nerves by the different forces or action of the rays, the red being 

 the most forcible, and the violet the least. 



