What We Learn from the Sun 



light is admitted into a darkened room, and a 

 prism be placed with its refracting angle down- 

 ward and horizontal, a vertical spectrum, having 

 its violet end uppermost, will be formed on a 

 screen suitably placed to receive it. 



But now let us consider what this spectrum 

 really is. If we take the light-waves correspond- 

 ing to any particular colour, we know from 

 optical considerations that these waves emerge 

 from the prism in a pencil exactly resembling 

 in shape the pencil of white light which falls 

 on the prism. They therefore form a small cir- 

 cular or oval image on their own proper part of 

 the spectrum. Hence the spectrum is in reality 

 formed of a multitude of overlapping images, 

 varying in colour from violet to red. It thus 

 appears as a rainbow tinted streak, presenting 

 every gradation of colour between the utmost 

 limits of visibility at the violet and red extremities. 



If we had a square aperture to admit the light, 

 we should get a similar result. If the aperture 

 were oblong, there would still be overlapping 

 images; but if the length of the oblong were 

 horizontally placed oblong, the overlapping; 

 would be less than when the images were square. 

 Suppose we diminish the overlapping as much as 

 possible; in other words, suppose we make the 

 oblong slit as narrow as possible. Then, unless 

 there were in reality an infinite number of images 

 distributed all along the spectrum from top to 

 bottom, the images might be so narrowed as to 

 not overlap; in which case, of course, there would 

 21 



