1891.] on An Astronomer'' s Worh in a Modern Observatory. 409 



telescope is rarely tranquil, sometimes it shines brightly in the centre 

 of the slit, sometimes barely in the slit at all, and the eye becomes 

 puzzled and confused. But the photographic eye is not in the least 

 disturbed ; when the star image is in the slit, the plate goes on record- 

 ing what it sees, and when the star is not in the slit the plate does 

 nothing, and it is of no consequence whatever how rapidly these 

 alternate appearances and disappearances recur. The only difference 

 is that when the air is very steady and the star's image, therefore, 

 always in the slit, the -exposure takes less time than when the star 

 is unsteady. 



That is one reason why the Potsdam results are so accurate. And 

 there are many other reasons besides, into which I cannot now enter. 

 What, however, it is very important to note is this, that we have 

 here a method which is to a great extent independent of the atmo- 

 spheric disturbances which in all other departments of astronomical 

 observation have imposed a limit to their precision. Accurate astro- 

 spectroscopy, therefore, may be pushed to a degree of perfection which 

 is limited only by the optical aid at our disposal and by the sensibility 

 of our photographic plates. 



And now I think we have sufficiently considered the ordinary pro- 

 cesses of astronomical observation to illustrate the character of the 

 work of an astronomer at night ; the picture should be completed by 

 an account of his work by day. But to go into that matter in detail 

 would certainly not be within the limits of this lecture. It is 

 better that I should in conclusion touch upon some recent remarkable 

 results of these day and night labours. It is these after all that 

 most appeal to you, it is for these that the astronomer labours, it is the 

 prospect of them that lightens the long watches of the night and 

 gives life to the otherwise dead bones of mechanical routine. 



Let us take first some spectroscopic results. To explain their 

 meaning let me remind you for a moment of the familiar analogy 

 between light and sound. 



The pitch of a musical note depends on the rapidity of the vibra- 

 tions communicated to the air by the reed or string of the musical 

 instrument that produces the note, a low note being given by slow 

 vibrations and a high one by quick vibrations. 



Just in the same way red light depends on relatively slow vibra- 

 tions of ether, and blue or violet light on relatively quick vibrations. 

 Well, if there is a railway train rapidly approaching one, and the 

 engine sounds its whistle, more waves of sound from that whistle 

 will reach the ear in a second of time, than would reach the ear were 

 the train at rest. On the ether hand, if the train is travelling at the 

 same rate away from the observer, fewer waves of sound will reach 

 his ears in a second of time. Therefore an observer beside the line 

 should observe a distinct change of pitch in the note of the engine 

 whistle as the train passes him, and as a matter of fact such a change 

 of pitch can be and has been observed. 



Just in the same way, if a source of light could be moved rapidly 



