16 THE CHEMISTRY OF THE SUN, [CHAP. 



of incidence the eye-glass required to be pulled much further out in 

 order to perceive the lines. If the eye-glass had the position proper 

 for seeing distinctly the lines in the red space, it was necessary to 

 push it in to see the lines in the violet space. If the aperture by 

 which the rays entered was enlarged, the finest lines were not easily 

 seen, and they disappeared entirely when it was about 40". 



"If it exceeded a minute the largest lines could scarcely be 

 seen. The distances of these lines and their relative proportions 

 suffered no change, either by changing the aperture in the shutter, 

 or varying the distance of the theodolite. The refracting medium 

 of which the prism is made, and the size of its angle did not 

 prevent the lines from being always seen. They only became 

 stronger or weaker, and were consequently more or less easily dis- 

 tinguished in proportion to the size of the spectrum. The propor- 

 tion even of these lines to one another appeared to be the same for 

 all refracting substances ; so that one line is found only in the blue, 

 another only in the red, and hence it is easy to recognise those 

 which we are observing. The spectrum formed by the ordinary and 

 extraordinary pencils of calcareous spar, exhibited the same lines. 

 The strongest lines do not bound the different colours of the spectrum, 

 for the same colour is almost always found on both sides of a line, 

 and the transition from one colour to another is scarcely sensible. 



"Fig. 6 shows the spectrum with the lines such as they are 

 actually observed. It is, however, impossible to express on this scale 

 all the lines and the modifications of their size. At the point A the 

 red nearly terminates and the violet at i. On either side we cannot 

 define with certainty the limits of these colours, which, however, ap- 

 pear more distinctly in the red than in the violet. If the light of 

 an illuminated cloud falls through the aperture on the prism, the 

 spectrum appears to be bounded on one side between G and H, and 

 on the other at B ; the light of the sun, too, of great intensity, and 

 reflected by a heliostat, lengthens the spectrum almost one-half. In 

 order, however, to observe this great elongation, the light between 

 c and G must not reach the eye, because the impression of that 

 which comes from the extremities of the spectrum is so weak as to 

 be extinguished by that of the middle of the spectrum. At A we 

 observe distinctly a well-defined line. This, however, is not the 

 boundary of the red, which still extends beyond it. At a there is 

 a mass of lines forming together a band darker than the adjacent 



