126 Notices of Books. [January, 



telescope. The measures are thus entirely differential, and there 



is no risk of errors having been introduced by undetected 



changes of zero. 



We may summarise the results of the observations in a few 



sentences: — Encke's comet gives a carbon spectrum. There is 



no polarisation to be detected in the light of the comet. The 



mass is certainly not less than that of an asteroid. The density 



of the supposed resisting medium in space, as computed from 



the retardation of the comet, is such that it would support a 



220 28 r 



column of mercury between — - and — ? of an inch in height. 



io 17 io-° 



There is some probability that the electric currents which give 



rise to auroras are propagated in a medium which pervades all 



space, and that the spectrum of the aurora is, in reality, the 



spectrum of that medium. It is not improbable that the tails of 



all large comets will be found to give spectra similar to that of 



the aurora, although additional lines may be present. 



In conclusion it may be said that, from the clearness of the 



detail, this pamphlet will be useful to the astronomical student. 



The Forces of Nature. A Popular Introduction to the Study of 

 Physical Phenomena. By Amedee Guillemin. Trans- 

 lated from the French by Mrs. Norman Lockyer ; and 

 Edited, with Additions and Notes, by J. Norman Lockyer, 

 F.R.S. London: Macmillan. 1872. 

 The progress of Physical Science is nowhere more clearly 

 apparent than in a comparison of the mode of producing its 

 records. The soberly bound volumes of half a century ago are 

 not more likely to be banished to the higher shelves of our book- 

 cases because the theories they expound are obsolete, than they 

 are to be superseded by the luxuriously printed and illustrated 

 books in which the philosopher of to-day declares the laws of 

 Nature according to his present lights. It is fit it should be so. 

 Delicate instruments and logical reasoning should have their 

 details drawn with a loving hand. Much of the science of yes- 

 terday lived grimly and darkly in its own study ; the science of 

 to-day throws its light upon all, and as a natural truth should be 

 shown as it appears, in its own attractive form. For instance, 

 why should not the diary of a journey through the realms of 

 light — " a fairy-like, enchanted world, a world of wonders, where 

 rubies, sapphires, topazes, and all kinds of precious stones send 

 forth their fires, where every object is of incomparable beauty 

 and splendour " — receive the most efficient ornament the aid of 

 art can impart. Such a luxury, if it is luxury, is a practical one, 

 for it raises in the mind of the student the enthusiasm which is 

 necessary to render him a lover of not only Nature, but, as well, 

 of Nature's laws. 



