388 Notices respecting New Books. 
polarization consequently exerts an electromagnetic force, 
exactly like an electric current flowing through a conductor 
in the same direction in which the displacement of positive 
electricity in an insulator takes place. 
The complete description of the experiments here briefly 
described, as well as of the numerous experiments made for 
the purpose of excluding all possible deception, will be given 
elsewhere. 
Iam now occupied with the construction of a piece of 
apparatus upon the same principle, which I hope will possess 
fewer defects and will be capable of producing greater deflec- 
tions than those described. Also I intend to put to experi- 
mental proof some other of the consequences of Faraday’s 
theory. 
Ty csablanion! it should be mentioned that with the suitably 
arranged apparatus I have repeated Rowland’s experiment 
described by Prof. Helmholtz as a test of its sensitiveness. 
The ebonite disk without coatings was charged by means of 
points. Upon reversal of the electrifications a deflection of 8 
to 10 scale-divisions took place each time. 
XLII. Notices respecting New Books. 
Lenses and Systems of Lenses, treated after the manner of Gauss. 
By Cuarizs PenpieBury, M.A. Cambridge: Deighton, Bell, & 
Cons 50p: 
1 the ordinary treatment of a lens it has been usual in our text- 
books to limit the discussion to the case of a ‘‘ thin” lens only. 
The result is, a number of interesting propositions are obtained 
which are practically false. Gauss, as Mr. Pendlebury states, in 
a paper which he communicated to the Royal Society of Gottingen 
(Dec. 10, 1840), shows “ how the solution of the (general) problem 
could be made to depend upon the determination, for each system 
and once for all, of four fixed points situated upon the axis of the 
system. These points having been determined, the complete solu- 
tion became a matter of simple algebra or geometry.” The method 
is applicable to any system of coaxial lenses, whatever their thick- 
nesses, provided the angle made by any ray with the axis, and the 
distance from the axis of the point of section of any surface, are 
small. This is, so far as our reading extends, the first attempt to 
introduce the treatment to English readers in a work intended for 
students; for this the author deserves their thanks. We have 
noticed a very few passages where an idiom reminds us of a Ger- 
man origin: some few more where there are slips which may 
mislead the student ; and some which appear to be incorrect. As, 
however, the author will no doubt have detected these and will 
remove them in the more extended treatment of the subject upon 
which we believe he is now engaged, we do not note them here. 
