101 
1883 .] Analyses of Books . 
any further. Though containing much that we cannot accept, it 
contains more that must be pronounced of exceeding interest, 
and no small stock of original and accurate observations. 
Physical Optics. By R. T. Glazebrook, M.A., F.R.S. Lon- 
don : Longmans and Co. 
This work forms one of the series of “Text-Books of Science ” 
which are being brought out by the Messrs. Longman. The 
author in his Preface explains the division of the science of light 
into Geometrical and Physical Optics, the latter of which deduces 
the laws of the propagation, reflexion, and refraction of light 
from a certain hypothesis, and explains at the same time certain 
phenomena which geometrical optics fail to account for. 
Even in this Preface the author refers to the difficulty of ex- 
plaining the existence of rectilinear rays of light on the principles 
of the received undulatory theory. He seems almost to apologise 
for the simplicity of the mathematical methods made use of, 
which he fears “ will appear long and clumsy to those who are 
acquainted with the differential and integral calculus.” At the 
same time many readers will think that he has made quite suffi- 
cient demands upon their attainments. 
In the first chapter we find an account of the conservation of 
energy, of light as a form of energy, of the Newtonian emission 
theory, and of the difficulties which led to its ultimate rejection. 
In the next chapter we have an explanation of the undulatory 
hypothesis, preceded by a notice of the assumed ether as the 
medium by which the light-vibrations are transmitted. Reasons 
for believing the existence of the ether are promised in a sub- 
sequent portion of the work. In the same chapter the author 
explains the rectilinear propagation of light, — a difficulty which 
Huygens, the originator of the undulatory theory, failed to over- 
come, and which was Newton’s chief objection. 
Chapter III. deals with reflexion and refraction, and explains 
the connexion between velocity, wave-length, and period. 
In the following chapters the author treats successively of 
prisms and lenses ; of interference ; the colours of thin plates ; 
diffraction, dispersion, and achromatism ; speCtrum analysis; ab- 
sorption and anomalous dispersion ; double refraction ; the 
reflexion, refraCtion, and interference of polarised light ; circular 
polarisation ; eleCtro^optics, and the velocity of light. 
Under the last head the author discusses the hypothesis of 
Messrs. Young and Forbes, that the blue rays travel more rapidly 
than the red, and concludes that in air, as in a vacuum, waves of 
all colours travel at the same rate. 
