Colours. 201 



of the telescope, that his attention was directed 

 to the wonderful effect which is produced by a 

 prism. 



The prism of the opticians is a triangular pris- 

 matic piece of glass, usually of the length of 

 about three inches. If a small hole ~F r fig. 77, 

 is made in the window- shutter, EG, of a dark 

 chamber, and a beam of light, SF, proceeding 

 directly from the sun (for the experiment will 

 only succeed when the sun shines), is made to 

 pass through the prism, ABC, an image of the 

 sun, PT, will be represented on the sheet of 

 paper, MN, fixed to the opposite wall. But 

 you will observe two very extraordinary cir- 

 cumstances attending this representation of the 

 sun. The first, that the figure is not round but 

 oblong; and, secondly, if you will observe the 

 figure in the plate, you will see that it is intended 

 to represent different colours, and in the real 

 image these colours will be found extremely 

 vivid. On measuring the image, which philo- 

 sophers have agreed in calling a spectrum. Sir 

 Isaac Newton found that, at the distance of 

 eighteen feet and a half from the prism, the 

 breadth of the image was two inches and a half, 

 and its length ten inches and one quarter, that 

 is, nearly five times its breadth. The sides were 

 right lines distinctly bounded, and the sides were 

 semicircular, as in the plate. From this it was 

 evident that it was still the image of the sun, 

 but elongated by some refractive power in the 



