THE SUN. 71 



and so on ; they would be found after projection all in- 

 deed lying in a line, but that line an interrupted one 

 consisting first of shot occupying a certain length ; then 

 an interval; then wheaten grains to a certain extent 

 another interval then sand, chaff, and so on. Now this 

 is by no means an inajDt though a coarse representation 

 of the constitution of the Prismatic Spectrum, When it 

 is formed by an extremely pure prism, and with certain 

 precautions (which need not here be detailed) to ensure 

 the perfect purity of its colours, it is found to be discon- 

 tinuous : that is to say, not a simple streak like a riband 

 of paper coloured from end to end by tints graduating 

 insensibly from red to violet, but like such a riband 

 marked, across its breadth, by perfectly black lines of 

 exceeding delicacy, yet some wider some narrower than 

 others ; and where these lines are, the paper is not illu- 

 minated at all. Into these spaces (for narrow as they 

 are, they have each a certain breadth) none of the light 

 dispersed by the prism falls. These lines, be it also 

 observed, are not occasional or accidental, but perma- 

 nent ; and belong to the sun's light as such. They 

 divide the spectrum into compartments as the boundary 

 lines between counties on a map divide the soil into 

 regions ; and each individual of these compartments 

 differs in other qualities besides colour from its neigh- 

 bours on either side; much as contiguous regions of a 

 country differ in soil and cultivation as well as in climate. 

 Tt is as if our assorted grains were distinguished not 

 only by bemg coloured according to tlieir respective 

 sizes; but each particular size and weight distinguished 



