December 27, 1900] 



NATURE 



205 



LETTERS TO THE EDITOR. 



\The Editor does not hold himself responsible for opinions ex- 

 pressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the writers of rejected 

 manuscripts intended for this or any other part of Nature. 

 No notice is taken. of anonymous communications. \ 



Relative Motion of the Earth and the Ether. 

 In recent issues of Nature and the Philosophical Magazine, 

 Profs. Larmor and Fitzgerald and Lord Kelvin have expressed 

 themselves as if satisfied that the negative result of the celebrated 

 Michelson and Morley experiment as to a relative motion of 

 earth and ether is genuinely decisive, and as if the only present 

 escape from the dilemma between this experiment and the bulk 

 of the evidence which necessitates a relative motion of earth and 

 ether is to be found in a possible change of the dimensions of 

 bodies caused by changes in their motion through ether. 



I should like to point out that a much less heroic alternative is 

 offered in an article of mine on relative motion of earth and ether 

 in the Phil. Mag., January, 1898. It is there shown that the 

 optical apparatus used by Michelson and Morley was probably 

 in a less sensitive condition than was assumed, and, therefore, 

 failed to determine the very minute distance which was the sub- 

 ject of measurement in their experiment. The argument of my 

 paper can be illustrated by the following considerations. Let 

 AB be a source of light, and let a'b' be a duplicate of it, for 

 example ab and a'b' may be the two images in Fresnel's pro- 

 duction of interference fringes 

 ^ j^ ~^ ^' by a biprism. The distance 



apart of the fringes at any 

 place is inversely proportional 

 to the distance between ab 

 and a'b'. 



But if, instead of a duplica- 

 tion such as AB and a'b', we 

 have one such as CD and c'd', 

 then an eye at XY can see 

 fringes whose width depends 

 on the angle at which CD is 

 inclined to c'd'. These are 

 two simple limiting cases with 

 correspondingly simple laws. 

 Michelson and Morley assume 

 that their apparatus gives them 

 the case of CD and c'd', but 

 there is nothing in the arrange- 

 ment of their apparatus to 

 ensure that the case shall not 

 be like that of EF and e'f', 

 where the two images have 

 suffered both the angular dis- 

 placement of CD and c'd', and the lateral displacement of ab 

 and a'b'. The law oCthe width of the fringes due to EF and 

 e'f' must be such as will include the laws of AB and a'b', and 

 of CD and c'd' as limiting cases. I hold, therefore, that the 

 Michelson and Morley experiment is vitiated by the assumption 

 that their apparatus gave them the full sensitiveness of the 

 cdc'd' position, whereas it really gave the unknown smaller 

 sensitiveness of the efe'f' position. 



They found that the displacement which they measured was 

 probably less than a fortieth of what might be expected as due to 

 the earth's orbital motion. I maintain that a possible alternative 

 to this conclusion is that the sensitiveness of their apparatus was 

 probably less than a fortieth of what they assumed it to be. 



On account of the great importance of the subject it is very 

 desirable that this experiment should be repeated with a definite 

 experimental measurement of the actual sensitiveness of the 

 apparatus employed. It would be indeed a great help in 

 astronomy if we could have Michelson and Morley apparatus of 

 known adequate sensitiveness in our observatories furnishing 

 continuous record of the earth's motion relative to the ether, from 

 which we could calculate the drift of the solar system, and 

 ultimately express all stellar movements with reference to the 

 ether. William Sutherland. 



Melbourne, November 6. 



tion that now familiar substance, ' ' liquid air. " In proof of this- 

 I may cite the following passages ; — 



(a) Georg. i. 404 (a meteorological passage, describing various 

 signs of fine weather). 



Apparet liquido sublimis in aere Nisus. 

 " Nisus (the hawk) is seen high up in the liquid air." 

 (3) ^neid vi. 202 (where the author is speaking of the sacred 

 doves which conducted /Eneas to the golden branch which was 

 to be his " open sesame " to the infernal regions). 



Tollunt se celeres, liguidumque per aera lapsae 

 Sedibus optatis geminaE super arbore sidunt. 



" Swiftly they soar aloft ; then, dropping through the liquid air^ 

 together settle on the wished-for tree." 

 Virgil does not seem to have made any scientific examination of 

 the substance he terms "liquid air"; but he must have noted 

 its transparency since he makes a point of the visibility of birds 

 through a thick stratum of it, and in the second passage he 

 evidently considers (whether from actual experiment or not must 

 be uncertain) that immersion in it had no effect on the " con- 

 ducting power " of Venus's doves. H. G. M. 



Virgil as a Physicist. 

 It seems to have escaped observation that, just as Homer 

 appears to have known of, and even given names to, the two 

 attendants or satellites of Mars, so Virgil is the earliest to men- volumes were tested 



NO. 1626, VOL. 63] 



The Sentinel Milk Steriliser. 



A RECORD of certain observations upon the action of the 

 "Sentinel" steriliser is given in your issue of December 13 

 (p. 166). 



The results of the temperature determinations are there stated 



as follows : — 



Bell Form. Cut-off Form. 



r Half pint 94° and 95° C. 95° and 93° C. 



Water-^ One pint 87-5° „ 89° 86° „ 87° 



i Two pints 87° ,, 86° 84° ,, 85° 



„.,, (Half pint 98° (frothing) 95° 



M'll^iTwopints 87° 84° 



and your reviewer adds : " It will thus be seen that there is- 

 considerable variation in the temperature." 



We trust that you will allow us to point out that the instru- 

 ment used was of the size designed for the sterilisation of twO' 

 pints of milk. An inspection of the above table proves that the 

 apparatus worked in a satisfactory manner even when one pint 

 only was placed in the vessel (the temperature readings being 

 87° to 89° in the bell form and 86° to 87° in the cut-off appar- 

 atus), and also that when the proper volume of liquid was 

 inserted, the variations in temperature were still further 

 reduced. 



As the apparatus is Jjurposely made in different sizes, and as 

 it was at no time supposedithat any one size would be used with 

 but one-quarter of the correct volume of liquid, the conclusion- 

 that " there is considerable variation in temperature" certainly 

 conveys a wrong impression to the reader. 



Your reviewer appears to doubt the wisdom of selecting 

 temperatures in the neighbourhood of 85° C. Now it is generally 

 acknowledged that Prof. Bang, of Copenhagen, is one of the 

 leading authorities on this subject, and therefore we give the 

 following passage from Scurfield's translation of Nocard's 

 "Animal Tuberculosis" (p. 73). 



"The extremely well-conducted experiments of Bang have 

 established the fact that the bacilli of tuberculous milk are 

 destroyed with certainty when the milk is heated to 85^ C. 

 for five minutes; between 75° and 80° they are sometimes- 

 killed, but not always ; they resist a temperature of 70', and 

 are afterwards able to render guinea-pigs and rabbits tuber- 

 culous when inoculated into the peritoneum ; but their vitality 

 is lowered, and they are no longer able to resist the action of 

 the digestive juices of those animals. At 60° their virulence 

 does not seem to be modified. Galtier has obtained results 

 almost identical with those of Bang." 



It is worthy of remark that one practical application of these 

 investigations is to be found in Denmark, where the law com- 

 pels all dairies that return unconsumed milk to "heat this milk 

 to 85° C. for a short time, and then rapidly cool the same. " 



(For the Cambridge Sentinel Manufacturing Co., Ltd.), 

 D. Berry (manager). 



In the directions for use issued with the " Sentinel " steriliser,. 



it is not stated that the apparatus should be filled or thereabouts ;. 



in consequence the variations in temperature with different 



It may not always be convenient or 



