1 30 KIRCHHOFFS APPARATUS. PART i. 



red near the least refrangible end of the spectrum, B 

 and C are in the orange, the very remarkable double 

 line D is in the yellow, b and E are in the green, F is 

 at the limit between the green and the blue, G is in the 

 blue, and the double line H is in the violet. 



The instrument used by MM. Bunsen and Kirchhoff, 

 though more complicated, is constructed on the same 

 principle as the preceding. A sunbeam transmitted by 

 a very narrow vertical slit passes through four prisms, 

 which disperse it so much, that if drawn on the scale 

 seen with the magnifying telescope which receives it, 

 the spectrum would extend over twenty feet. By means 

 of a micrometer screw, the telescope can be turned 

 round a vertical axis, and as the dark lines come 

 successively under the cross wires in its eye-glass they 

 are seen to pass over a graduated scale, so that the 

 distances between two thousand of them have been 

 measured in millimetres with unerring accuracy, but 

 that is only a small part of the whole. When viewed 

 through the telescope, the retina of the eye is the 

 screen on which this wonderful spectrum falls, crossed 

 by innumerable dark rayless lines of various breadths 

 and intensities. Black bands given by the inferior 

 refraction of one prism are here resolved into nume- 

 rous dark lines as fine as a spider's thread. 



Mr. Glaisher during his tenth scientific balloon ascent 

 devoted his attention for a time almost entirely to the 

 dark lines on the solar spectrum. At a height of about 

 four miles and a half, they were almost innumerable ; 

 all he had seen on the earth were there, and many more. 

 The nebulous lines H were both seen, the spectrum was 

 a good deal lengthened at the violet end, and at the 

 red end the line A was visible. The light from the 

 sky near the sun gave a shorter spectrum ; the lines 

 were only visible from B to G. 



Besides these cosmical or permanent lines, Sir David 



