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adopted by spectroscopists is the tenthmetre, i.e., the ten 

 millioueth part of a millimetre, a millimetre being 0'03937 of an 

 inch. The length of the longest wave that we can see in the red 

 is 7,600 of these small units, and of the shoitest in the violet is 

 3,932 of the same miit. If a body giving out light is advancing 

 towards us, more waves will reach the eye in a given second 

 or, what is the same thing, relatively to us the length of the 

 waves will be shorter, and whatever is true of bodies advancing 

 is true in an opposite sense of bodies receding. Take, then the 

 case of a glowing cloud of hydrogen swiftly advancing in our 

 line of sight on the sun. Suppose we are observing it on its 

 blue line. The waves of light giving this line have a certain 

 definite length, namely, 4,860 tenth-metres. What will happen ? 

 Since the body is advancing, the length of the waves will be less 

 than 4,860 units, hence the bright blue line will be displaced 

 from its proper position and thrown down in the scale towards 

 the violet. If the body had been receding, the line would have 

 been thrown towards the red. A change of one-tenth metre in 

 the position of the blue line of hydrogen towards the violet would 

 indicate an uprush of this gas at the rate of 38 miles a second. 

 The spectroscope, then, is not only an instrument for finding out 

 what the heavenly bodies are made of but can even be employed 

 to gauge their velocities in the line of sight. We say in the line 

 of sight because, as a moment's reflection will show, the method 

 is inapi)licable to motions at right angles to the line of sight. 

 Hence on the body of the sun it measures the velocity of the gas- 

 streams which are rushing up or down, while at the limb it will 

 measure not the up and down rushes, but those only towards or 

 away from us. But, as we have already seen, up and down 

 rushes or vertical motions on the limb can be detected in the 

 open slit of the spectroscope, and measured by an ordinary 

 micrometer. By such means of observation velocities of sixty, 

 eighty, and of a hundred miles a second have been often detected 

 in masses of hydrogen. A phenomenal instance occurred on 

 August 3, 1872., as observed by Young, when the same iiame 

 gave in one part indications of an uprush of 250 miles a second, 

 and in another part a downrush of 230 miles a second. More 

 wonderful still. Young noted in this outburst three times of 

 special intensity, and at these precise instants the magnets at 

 Greenwich and Stonyhurst gave responsive deflections. The 

 greatest average velocity in a flame ever observed by Secchi was 

 about 230 miles a second, in the outburst described above of 

 October 16, 1871. 



