[1] It will be noted that Mr. Bolton pronounces the geometry of space to be Euclidean in the absence of gravitational fields, not that of space-time. This is in accord with what was pointed out on page 161.--Editor. [2] The author here comes perilously close to ascribing to this "contraction" the sort of physical reality which it does not possess. See page 96.--Editor. [3] Dr. Davis went rather fully into the algebra of the Michelson-Morley experiment. But Dr. Russell has covered the same ground in a form somewhat more advantageous from the typographical viewpoint, and the point is not one which it is profitable to discuss twice; so we eliminate this part of Dr. Davis' text.--Editor. [4] This statement is objectionable, as explained in Chapter IV.--Editor. [5] A. Einstein: Die Grundlage der allgemeinen Relativitätstheorie. Ann. d. Physik. 4, vol. 49, page 776. [6] At this point we have again used the blue pencil on Dr. Davis' text, his discussion of the three observational tests of the General Theory adding nothing to Dr. Pickering's.--The Editor. [7] Commander McHardy uses the term "event" in a sense somewhat different from that seen in a majority of the essays. He reserves for the four-dimensional element--the instant of time at a point in space--the name "point-event"; and the term "event" he applies to a collection of these forming, together, an observable whole. An actual physical happening, like a railroad wreck or a laboratory experiment, it will be realized is of the latter sort, occupying an appreciable region of space rather than a single point, and an appreciable interval of time rather than a single second. To the element, the "point-event" of Commander McHardy's essay, this bears the same relation that the geometer's solid bears to his point. This comment is in no sense to be taken as criticism of Commander McHardy's terminology, which rather appeals to us; we make it merely to guard against confusion in the reader's mind.--Editor. [8] This paragraph is the result of an editorial revision of the author's text, designed to retain the substance of his presentation, while tying up what he has to say more definitely with the preceding essays, and eliminating the distinction between finite and infinitesimal intervals, which we believe to be out of place in an essay of this character. We will not apologize to our mathematical readers for having used finite and differential notation in the same equation, in violation of mathematical convention.--Editor. [9] Although gravitational force in a small region can be imitated or annulled by accelerating motion, there remains the disturbing influence of gravitational matter already referred to and expressed in the fabric curvature. It is this that defines how unique tracks run, or rather, how bodies progress.--Author.