Notices respecting New Books. 407 



experimenter cannot fail to notice and be perplexed by), nor, in the 

 case of lenses, of chromatic aberration. 



The student is taught how to make a spectroscope for himself 

 which, though rough, is all that is absolutely necessary to show the 

 distinctive spectrum-lines of such elements as sodium, lithium, 

 calcium, <fcc. The description of this little apparatus, and of the 

 method of using it, occupies many pages and is too long to quote 

 here ; it may, however, be stated that it consists of a single disul- 

 phide-of-carbon prism, which is made from the wider part of a 

 lamp-cylinder cut obliquely at both ends and closed by glass plates, 

 with an aperture on the upper side for pouring in the liquid. 

 This is placed at one end of a long wooden box with square 

 section, the further end of which is closed by a thin plate of wood 

 with a narrow vertical slit in it. Following the directions given, 

 the student will have no difficulty in arranging apparatus for exhi- 

 biting the reversion of the bright lines in the spectrum of an 

 element, and in apprehending the cause of such reversion. 



We find nothing about the phenomena of Interference or Dif- 

 fraction. This is scarcely to be wondered at, since experiments on 

 these subjects demand the use of more elaborate instruments than 

 the author contemplates his students to be in possession of. His 

 plan is throughout to confine himself to the exposition of such 

 phenomena and principles as can be reproduced and exemplified 

 by the student with such apparatus as he can construct for himself. 

 The student is not expected to make measurements of any kind; 

 he is not supposed to have arrived at such a state of proficiency in 

 his subject. Indeed no measuring-instruments are described or 

 even mentioned in any part of the book. We think, however, it 

 would not have been inconsistent with the plan of the work had 

 some of the elementary experiments on the polarization of light 

 been given. 



In the treatment of Electricity the same order is adopted as in 

 the ordinary books on Physics. This portion of the work, not less 

 than the rest, is characterized by copious illustration of principles 

 and by many-sided hints and suggestions for insuring the suc- 

 cess of the experiments described. In the main we agree with 

 the writer of the Preface, that though the instructions may some- 

 times appear to readers unaccustomed to experimental work need- 

 lessly minute, yet it will appear in actual trial that the apparently 

 small matters to which attention is sometimes called are just such 

 as make the difference between success and failure. At the same 

 time the thought forces itself upon us as we read through these 

 many pages of instructions, that the author would have done wisely 

 had he sometimes curtailed them, and so left space for many useful 

 and important matters which he has been obliged to omit. We 

 should like to have seen explained and illustrated the law of in- 

 verse squares as applied to electrical and magnetic attractions and 

 repulsions, something about the torsion-balance, Faraday's theory 

 of induction, specific inductive capacity, thermo-electricity, the 

 tangent-galvanometer, electrical resistance, &c. Of none of these 

 is any mention made. 



