of certain variably colored Stars. 191 
a 
system is moving through space; but the difficulties in the way, 
of gaining any accurate comparison of those velocities are very 
at. 
In reflecting upon the undulatory theory of light I have been 
quite unable to conceive how the luminiferous ether could 
“tremble laterally’’ as the phrase is, causing vibrations transverse 
to the line of propagation, without a direct relation existing 
between those lateral vibrations and the forward impulses b 
er of impulses per second, and its 
impression of color therefore must be correspondingly modified. 
the retina and the source of light rapidly separate from each 
other, the number of impulses striking the retina must on the 
other hand be diminished, producing the corresponding change 
lor 
in the perceived color. : k, 
_, Now, if we imagine a star emitting white light to approach us 
in an orbital movement at a sufficient rate of ‘speed, its light 
should appear to us reddish, changing at the perigee into white, 
anging again into blueish as the star departs, and again into 
white at the apogee. There are, however, variable stars whose — 
passing from one 
Color to its complimentary, and back again, with periods of white : 
Tight intervening. The bin uently 
complimentary to each other, should, under the proper circum- 
