June i6, 1892] 



NATURE 



ik. 



'65 



direction of the ray, whilst CP is the wave normal. In the 

 above figure, 



sin e _ SC c ,, 



sin fl "~ rP ~ V ~ "' constant of aberration. 



The velocity along the path of the ray is SP. Calling this 

 velocity V, we have 



V r= V cos 6 + z* cos 9. 

 The path of a ray is determined by the time of journey being a 

 minimum, and the formula 





a mmimum, 



is the equation to a ray, where A and B are the extremities, and 

 ds an element of the path. If the ether be moving, V must be 

 substituted for V, and we get — 



H\^ 



cos 6 + f cos 9 

 This integral can be written exactly — 



a minimum. 



T' — {'^ cos e C V cos fl , 



T cos fl 



cos 



.ds. 



The last term is the only one involving the first power of ether 

 drift, and it vanishes in case there is a velocity potential ; for, 

 since v co?, 9 = J, where <p is the velocity potential, it may be 



written y^Zli\ ; and so its value depends only on the end 



points and not on the path. If these points are the same i.e. 

 the contour is closed, it becomes zero, and reconciles all the ex- 

 periments hitherto made. It must be admitted, however, that 

 if a is not a constant, the question is again opened, but there is 

 no reason to suppose it can vary in the same horizontal plane 



V 

 If the medium be changed, V becomes , and, in order to re- 



tain the same velocity potential in the changed medium, v must 



become -~, which is Fresnel's law. Hence Prof. Lodge pointed 



out that the velocity potential condition includes Fresnel's law 

 as a special case. It can, in general, be inferred that no first 

 order optical effect due to terrestrial motion can exist in a detect- 

 able form. It is always compensated by something else. 

 Quantities of the second order of magnitude must, therefore, be 

 attended to. From the first equation above, it follows that ' 



x/i 



o- sin- 9, 



and the time of journey in moving ether is given by 

 J, _ -p Vl — t? sin*"^ 

 I - a- 

 where T is the time if everything were stationary. This is, in brief, 

 the theory of Michelson's recent experiment. If the light travels 



along the ether drift, fl = o and T, = -^~.; whilst if fl = 00°. 

 ' I - a* ^ ' 



T 

 T'=: -p=^. Therefore the velocity along the ether drift 



should differ from that across the drift in the ratio of \^i ^^^ : i. 

 This point has been very carefully tested by Michelson, but 

 nothing approaching to a quarter of the theoretical effect was 

 observed. His negative result would seem to preclude any 

 relative motion, even irrotational, and shows that the ether is at 

 rest relative to the earth's surface. On the other hand, the 

 author (Dr. Lodge) had recently made experiments on the in- 

 fluence of rapidly-rotating steel disks on the ether, which prove 

 that the ether is not affected by the motion of contiguous matter 

 to the extent of 1/200 part of the velocity of the matter. Thus, 

 these two experiments are at present in conflict. Prof. Fitz- 

 gerald has suggested a way out of the difficulty by supposing the 

 size of bodies to be a function of their velocity through the ether. 

 Returning to the statements which have been made of Fresnel's 

 law, Glazebrook has shown that actual extra-density of ether is 

 not necessary, for, if the virtual mass be altered, the same results 

 follow ; all that is required is a term depending on the relative 



NO. 118 I, VOL. 46] 



acceleration of ether and matter. To modern ideas the loading 

 of the ether by the presence of matter is most likely to be 

 correct, and the observed effects of relative motion are regarded 

 as the results of secondary reactions of matter on ether. On 

 this view, the ether of space may be wholly unaffected by the 

 motion of matter. On the vortex ring theory of matter, it is not 

 unnatural to suppose that the ether in its neighbourhood should 

 be only affected irrotationally by its motion. And if the 

 velocity potential be granted, nothing of the nature of viscosity 

 being admissible, the results of all the interference, refraction, 

 and aberration experiments could be predicted, and the whole 

 theory is as simple as it can possibly be. The only trustworthy 

 experiment ever made which tends against this view is that of 

 Michelson. The author surmised that this must somehow be ex- 

 plained away. In reply to a question from Prof, Ayrton, Dr. Lodge 

 said that when air was substituted for water in Fizeau's experi- 

 ment no effect was observed. This might have been expected, 

 for the difference in the times of journey by the two paths 

 depended on '^ .^ , and as fj. is nearly unity for air, the air 



effect is too small to see. [In Hoek's interference experiment it 

 might be said that the effect of ether moving in stationary water 

 is balanced by that of the ether moving in stationary air ; but 

 while motion of water itself would disturb the balance, motion 

 of air would do nothing appreciable. The only kind of motion 

 that could display an optical effect is rotational motion, or 

 motion of layers at different speeds, not a simple uniform drift. 

 Prof. J. V. Jones asked how the Fizeau experiment could be 

 expressed on the loaded ether theory ; for, since the speed of 

 matter affects the velocity of light, it seemed to involve a 

 directional loading. A mere extra-density term, or acceleration 

 coefficient, will not explain this; it seems to require a co- 

 efficient of a velocity term. This question has been hinted at 

 by Lord Rayleigh, who points out (under the heading 

 •' Aberration," Nature, xlv.fp, 499) that the rate of propaga- 

 tion of waves on a loaded string will be affected by a travelling 

 of its load. The question is not perfectly simple, and the 

 analogy not complete. A good deal depends on the nature of 

 the connection symbolized as " loading." >] 



Royal Microscopical Society, May 18 — Dr. R. Braith- 

 waite. President, in the chair. — Mr. R. T. Lewis, in his 

 paper on the process of cviposition as observed in a species 

 of cattle tick, said that the tick was observed under a low 

 power ; after some time the head with the extended rostrum 

 and palpi was retracted, producing a deep depression, the softer 

 adjacent portions of the ventral surface between the basal joints 

 of the first pair of legs being drawn over the margin. Parts 

 surrounding the depression changed colour, and a white vesicle 

 appeared upon the lower internal wall. The palpi separated, 

 so that they rested on each side of the vesicle. A membranous 

 body, glistening with mucus, was protruded from the cavity, 

 from the lateral extremities of which two papillae were thrown 

 out, extending across the depression. The vesicle was then 

 elongated and embraced by the papilla; ; through its walls an 

 egg was seen in motion, which, being delivered into the grasp 

 of the papillae, the ovipositor at once retracted. The papillae 

 closed round the egg, covering it with an albuminous secretion, 

 and withdrew, leaving it suspended from the under surface of 

 the dorsal plate. The palpi closed together until in contact 

 with the rostrum, the head elevating, clearing the egg out of 

 the depression, leaving it adhering to the outer margin : the 

 entire process of laying each egg occupying a period of 2 min. 42 

 sec. Mr. A. D. Michael remarked that the word "head " was 

 somewhat misleading, because these animals had no heads in 

 the sense in which the term applied to insects, but the whole 

 movable organ was really the rostrum. — Mr. E. M. Nelson 

 read a note on penetration in the microscope, showing that for 

 his own sight the penetrating power was only one-seventh of. 

 that given by Prof. Abbe, whose myopic sight accounted for 

 the difference in the estimate. — Mr. Nelson also read a note 

 on rings and brushes of crystals, for the observing of which a 

 petrological microscope was generally thought to be necessary. 

 This was not essential, as it was really a telescopic object. 

 All that had to be done was to convert the microscope into 

 a telescope by placing an objective inside the tube of the 

 instrument. 



Geological Society, May 25.— W. H. Hudleston, F.R.S., 

 President, in the chair. — The following communications were 



• Note by 0. J. L. 



