92 PLEASANT WA YS IN SCIENCE. 



middle of the yellow part in one case, or to the middle of 

 the blue part in the other. The dark line would be quite 

 notably shifted in either case. With the actual stellar 

 motions, though all the lines are more or less shifted, the 

 displacement is always exceedingly minute, and it becomes 

 a task of extreme difficulty to recognize, and still more to 

 measure, such displacement. 



When I first indicated publicly (January, 1868) the way 

 in which Doppler's principle could alone be applied, two 

 physicists, Huggins in England and Secchi in Italy, were 

 actually endeavouring, with the excellent spectroscopes in 

 their possession, to apply this method. In March, 1868, 

 Secchi gave up the effort as useless, publicly announcing the 

 plan on which he had proceeded and his failure to obtain 

 any results except negative ones. A month later Huggins 

 also publicly announced the plan on which he had been 

 working, but was also able to state that in one case, that of 

 the bright star Sirius, he had succeeded in measuring a motion 

 in the line of sight, having discovered that Sirius was receding 

 from the earth at the rate of 41^4 miles per second. I say 

 was receding, because a part of the recession at the time of 

 observation was due to the earth's orbital motion around the 

 sun. I had, at his request, supplied Huggins with the 

 formula for calculating the correction due to this cause, and, 

 applying it, he found that Sirius is receding from the sun at 

 the rate of about 29^ miles per second, or some 930 millions 

 of miles per annum. 



I am not here specially concerned to consider the actual 

 results of the application of this method since the time of 

 Huggins's first success ; but the next chapter of the history 

 of the method is one so interesting to myself personally that 

 I feel tempted briefly to refer to details. So soon as I had 

 heard of Huggins's success with Sirius, and that an instrument 

 was being prepared for him wherewith he might hope to 

 extend the method to other stars, I ventured to make a 

 prediction as to the result which he would obtain whensoever 

 he should apply it to five stars of the seven forming the so- 



