148 
POPULAR SCIENCE REVIEW. 
spectroscopes have not only different dispersive power, but give 
spectra in which the distances between the fixed lines are not 
proportional inter se. Let not the observer, therefore, fall into 
the error of imagining (as we believe a well-known foreign 
spectroscopist did) that the lines of a star’s spectrum have 
changed their place, when the apparent difference is in reality 
due to the “ irrationality of dispersion.” In fact, every observer 
must thoroughly master the peculiarities of his own spectroscope 
before pretending to discuss the results obtained by means of it. 
He should know the exact positions of all the principal lines 
and groups of lines shown by it, so that when he has observed 
any line in a star’s spectrum he may know approximately the 
true position of that line with reference to the solar spectrum. 
He will then know what are the substances whose spectra should 
be compared with that of the star, in order to ascertain if this 
line coincide with one of those belonging to known elements. 
It need hardly be said that, as the image of the star is to be 
kept exactly within the slit, it is an absolute necessity that the 
telescope should be an equatorial, accurately driven by clock- 
work. 
It is also necessary that the telescope should be one of large 
aperture, as otherwise none but the brightest stars could be 
examined with powerfully dispersive apparatus. On this account 
it is probable that reflectors will be largely used by spectro- 
scopists, since achromatic refractors of suitable aperture are so 
much more expensive than reflectors of corresponding illumin- 
ating power. It happens, fortunately, that Mr. Browning, who 
has paid so much attention to the improvement of reflecting 
telescopes, appreciates also most thoroughly the position which 
spectroscopic analysis is about to take up as a means of astrono- 
mical research. The spectroscopes he constructs for use with 
the telescope correspond in all essential respects with the form 
above described. 
For special purposes great dispersive power may be required 
in the observation of stellar spectra. Fig. 9 shows an arrange- 
ment by which ]\Ir. Huggins has obtained great dispersion 
without an amount of deviation which would cause the pencils, 
after passing through the prisms, to cross those from the 
collimator. In this instrument a is an adjustable slit, h an 
achromatic collimator, c the telescope with which the spectrum 
is observed, d and e are direct-vision prisms,/, rj, and h simple 
prisms. The total dispersive power of this instrument is 
equivalent to that from six and a half prisms of 60 degrees ; or, in 
other words, corresponds to what would be given by a set of 
simple prisms which would caiLse a deviation through more than 
four right angles. 
This paper would be incomplete if we did not touch upon the 
