DR. OLIVER LODGE OR ABERRATION PROBLEMS. 729 
Page. 
Section 68. Effect of drift on rays. Error of reflexion. Possibility of detecting it . . 797 
69. Error of refraction. 798 
70. Verification by Stoke.s’ method, pushed to second order of miniitiEe .... 800 
71. Possibility of detecting an effect due to error of refraction. 800 
72. Effect of reflexion on wave-length". 801 
7.3. Effect of reflexion on phase. 801 
74. Effect of reflexion on energy. 802 
75. Possible effect of pressure of light. 802 
76. Direction of motion of a place on the earth. 803 
Electrical Methods. 
77. Electrical methods, like optical methods, can only detect second order of aberra¬ 
tion magnitudes as caused by general ethereal drift, i.e., by motion of 
apparatus and observer through space. 804 
1. The nature of the connexion between ether and gross matter is one of the most 
striking- physical problems which now appear ripe for solution, and as a preliminary 
to the undertaking of fresh experiments I propose to review the subject in order to 
realize our present position in connexion with it. 
The subject may be attacked either optically or electrically. It first prominently 
presented itself historically in connexion with the earth’s motion through space and 
the finite velocity of light; and it will be convenient to consider the matter first on 
this side, and to realize precisely what ought to be expected to happen on any simple 
hypothesis concerning the ether; working it out, however, in most cases Avith accuracy,* 
and by no means ignoring as negligible small quantities of the second order (hun¬ 
dred-millionths), which Mighelson has practically shown to be nowadays within the 
limits of highly-refined observation. 
Necessary Hypotheses or Postulates. 
2. There are one or two hypotheses regarding the ether so elementally obvious that 
they may be regarded as almost axiomatic, such as the following:— 
(i.) In interstella.r space the ether is free; that is, its properties in no way depend 
on the existence or motion of gross matter. It may, therefore, be called at rest. 
Whether it is in absolute rest or not appears to be a question which can hardly be 
put into an intelligible shape. If it be moving relatively to itself, we have in those 
regions no obvious means of ascertaining the fact. But just as it is natural to assume 
that its properties in free space are uniform, so it is natural to assume that its motion 
there, whatever it is, is perfectly uniform, and it may be defined as absolute rest. 
When I speak of the ether anywhere as “ stationary,” I mean stationary with respect 
* Whenever equations are approximate only, the symbol is used instead of the symbol =. 
MDCCCXCIII. — A. 5 A 
