156 HERSCHEL AND HIS WORK 



an unknown centre. He did not find it, but he be- 

 lieved, as we have already seen, first, that the sun was 

 moving among the stars, and second, that it was 

 moving towards a spot in the constellation Hercules in 

 the northern sky. 



As the sun is the source of light and heat, and both 

 of them had to be considered in his observations, it 

 was natural that Herschel should turn his thoughts to 

 the solar spectrum, as we call what is commonly spoken 

 of as the rainbow. A glass prism produces the same 

 effect on a beam of sunlight as a raindrop or a cloud 

 curtain composed of millions of them: it divides or 

 decomposes the white light of one sun into that of 

 seven suns of different colours, red, orange, yellow, 

 green, blue, indigo, violet, and it also bends or refracts 

 them from the straight line the sunbeam would other- 

 wise pursue. The red is the least bent, the violet 

 most. By the refraction or bending is meant what is 

 seen by thrusting one half of a walking-stick into 

 water, and keeping the other half out of it in the air. 

 But it happened that in shielding his eye from the 

 sun when looking at its disc through a telescope, 

 Herschel had used glass of various colours to dim the 

 glare and heat. This experience was fatal to the use of 

 glass coloured red. "I began with a red glass," he 

 says, "and, not finding it to stop light enough, took 

 two of them together. These intercepted full as much 

 light as was necessary ; but I soon found that the eye 

 could not bear the irritation, from a sensation of heat, 

 which, it appeared, these glasses did not stop. I now 

 took two green glasses : but found that they did not 

 intercept light enough. I therefore smoked one of 

 them: and it appeared that, notwithstanding they 



