248 Notices of Books. (April, 
with spectrum analysis in its application to terrestrial sub- 
stances; and the third, with the application of the analysis to the 
heavenly bodies. 
We need say little about the first part of the work. It is ob- 
vious that the book would have been incomplete without: an 
account of the chief artificial sources of light and heat; and the 
fifty pages devoted to the description of the Bunsen burner, the 
magnesium light, the oxyhydrogen flame, the lime-light, and the 
electric light, cannot be regarded asin excess of the requirements 
of the practical student of spectroscopic analysis. 
With the second part we cannot express ourselves wholly sa- 
tisfied. The explanation of the fundamental principles of 
prismatic analysis is insufficient, and even in part unsound. 
Indeed we cannot but notice that where Dr. Schellen approaches 
a subject involving mathematical considerations, his conceptions 
are wanting in clearness and exactness. His explanation of the 
‘‘indivisibility of the pure colours of the spectrum” may be cited 
as an instance, and in some respects a very remarkable instance. 
The subject matter is obsolete, to begin with, since it relates in 
reality only to Newton’s recognition of the fact that light from a 
given part of the spectrum cannot be resolved into light of other 
colours,—a fact of no importance in the present position of the 
science. But Dr. Schellen, in attempting to explain this, falls 
into the mistake of asserting what is untrue, though it would be 
unquestionably most important if true. He says, ‘If a small 
round hole be made in the screen, in any portion of the image of 
the spectrum,—the extreme red, for instance,*—a red ray passes 
through it, and appears on the opposite wall as a round spot of 
red light. If a second prism be interposed in the path of the 
ray that has passed through the screen, the ray will suffer a 
second refraction, and the image be thrown upon another place 
on the wall: this new image, however, is simply red like the in- 
cident ray, and by a careful adjustment of the prism shows no 
elongation, but appears perfectly round.” ‘This, of course, is 
utterly erroneous; were it true, our spectroscopists would gain 
nothing by employing more than a single prism in any of their 
researches. In extending the reasoning, Dr. Schellen introduces 
a figure representing the action of a second prism on the central 
colour of the spectrum ; but in ¢hzs figure the second prism is so 
placed as to reverse the action of the first, the contrary being 
the case in the figure illustrating the former part of his reasoning. 
In the case illustrated by the second figure a round spot of light 
would naturally be obtained, the second prism restoring to paral- 
lelism the rays which the first prism had caused to diverge. 
But although in dealing with theoretical considerations Schellen 
is thus not unfrequently inexact, the description of the practical 
* The reference in the text is to Fig. 28. It should be Fig. 32, as will appear 
by a reference to the first German edition. 
