330 Dr. Foi'Ster on the dispersive Power 



when brought to its proper focus, we gain in the duller and 

 more easily observable expanded spectrum. This me- 

 thod, in achromatic telescopes, does not separate the different 

 ravs so as to produce an oblong and coloured spectrum; 

 but on the contrary, the large spectral disk which I have de- 

 scribed, corresponds in its general colour with the colour of 

 the star viewed with the naked eye; differing only in this, that 

 the light not being intense or dazzling, we are enabled to ob- 

 serve minuter differences in the colour of different stars viewed 

 in this manner, than we could do if we let the rays arrive to 

 the focus. This method is likewise useful for observing more 

 accurately the permutations of colour noticed in the fluctua- 

 tions of some stars, which I have before described. Since my 

 last paper I have noticed more minutely the particular colours 

 of the alternate changes in several stars, and have also ob- 

 served some strange and unaccountable coruscations and ap- 

 parent motions in the luminous striae of brilliant white light, 

 which sometimes seems to intersect this pale and expanded 

 spectrum described above, either in the form of lines, of radii, 

 or of reticular intersections, leaving frequently dark spaces 

 and unilluminated interposing lines. These coruscations have 

 very much the appearance of some effects produced in arti- 

 ficial electricity, or of certain coruscations of the Aurora 

 Polaris. 



I hope on a future occasion to be able to communicate some 

 further experiments on this phenomenon, as well as on an- 

 other mode of observing stars with prisms, so as to get the 

 black lines of non-illuminating rays at the regular intervals. 

 But at present I am desirous to confine myself to the precise 

 object of this paper, which is, to show the comparative refran- 

 gibility of different star-light, and the necessity of tables of 

 correction for each particular star of any considerable magni- 

 tude. I have noticed that the red colour, which alternates in 

 the fluctuation, generally, but not in every case, alternates with 

 the ordinary or general colour of the star, stated in the fourth 

 column. This alternation is not distinctly observed in stars 

 near to the zenith, but begins to be perceived very clearly at 

 about 48° of altitude, and is greatest at between 10° and 15° 

 above the horizon. When stars which are much subject to 

 this alternation of colour are viewed at the above altitudes in 

 the method above described, that is, by observing them before 

 the rays come together in the focus, we not only observe more 

 accurately and in an enlarged and less intensely luminous disk, 

 the same phaenomena that we can view with the naked eyes, 

 namely, the sudden change of the whole star to deep red ; but 

 we may also distinguish that this red colour is not instanta- 

 neous! v 



