40G 



SLIPHER— NEBULA. 



In Table I. are given the velocities for the twenty-five spiral 

 nebulae thus far observed. In the first column is the New General 

 Catalogue number of the nebula and in the second the velocity. 

 The plus sign denotes the nebula is receding, the minus sign that it 

 is approaching. 



Generally the value of the velocity depends upon a single plate 

 which, in many instances, was underexposed and some of the values 

 for these reasons may be in error by as much as lOO kilometers. 

 This however is not so discreditable as at first it might seem to be. 

 The arithmetic mean of the velocities is 570 km. and 100 km. is 

 hence scarcely 20 per cent, of the quantity observed. With stars 

 the average velocity is about 20 km. and two observers with dif- 

 ferent instruments and a single observation each of an average star 

 might differ in its velocity by 20 per cent, of the quantity meas- 

 ured. Thus owing to the very high magnitude of the velocity of 

 the spiral nebulae the percentage error in its observation is compar- 

 able with that of star velocity measures. 



Since the earlier publication of my preHminary velocities for a 

 part of this list of spiral nebulae, observations have been made 

 elsewhere of four objects with results in fair agreement with mine, 

 as shown in Table II. 



TABLE II. 

 Velocities of Nebul.^ by Different Observers. 



observers. 



Slipher, mean from several plates. 

 Wright, Lick Observatory, one plate. 

 Pease, Mt. Wilson Observatory, one 



plate. 

 Wolf, Heidelberg, one plate approx. 



Pease, Alt'. Wilson, from bright lines. 

 Slipher, from bright lines. 



Slipher, from dark and bright lines. 

 Pease, from two bright lines. 

 Moore, Lick Observatory, from three 

 briglit lines. 



Slipher. 



Pease, Mt. Wilson Observatory. 



