yune I, 1876] 



NA rURE 



119 



ment. We have two electrical keys, one at the further 

 end intended for making what is called the signal, and 

 one here for breaking, which is placed close to the person 

 who is to be experimented upon. Mr. Page, at any 

 moment he likes, will act upon me by sending an induc- 

 tion flash through my tongue. I shall arrange the elec- 

 trodes so that they shall be against the tip of my tongue, 

 and at the moment I feel that flash I shall place my 

 finger on the key. Then the clockwork being in motion 

 at the same time, we shall see by the length of the 

 depression in the tracing the duration of the process. 

 If we take different sorts of signals, or if the person 

 to be experimented upon is in difiercnt conditions, the 

 time will be very difl^erent. Thus we may compare the 

 result which will be produced when I am attending and 

 expecting the signal with the result which will be pro- 

 duced when I am not attending or expecting the signal ; 

 or, on the other hand, I may compare those results 

 with that which will be produced when I am expect- 

 ing it, but Mr. Page, instead of giving it at the time I 

 expect it gives it me at a different time ; in that case the 

 time occupied would be longer than in either of the other 

 two cases. A great variety of different cases can be in- 

 vestigated in this way in which we measure the total period 

 occupied in the reflex. The arrangement is perfectly 

 simple. You see when Mr. Page presses on his key, 

 which is the signal key, that a lever is set in vibration and 

 makes a tracing, and at the same moment the voltaic 

 current is made and the coil is acted upon inductively ; 

 the 'result is that an induction flash passes through my 

 tongue which I feel, and the moment I feel it I break the 

 current. Consequently the time between the moment at 

 which Mr. Page makes the cu' rent by closing his key and 

 the moment at which I break the current by placing my 

 finger on my key, gives us precisely the time which is 

 occupied by the reflex process. We will make two experi- 

 ments, first, with the signal expected, and then unex- 

 pected ; that is, in the one case I shall be on the qui vive, 

 and on the other I shall not be so. (The experiments 

 were made accordingly.) We shall now repeat the pro- 

 cess, so that instead of my receiving the information of 

 the making of the current by means of the excitation of 

 my tongue, the signal shall consist in my hearing the 

 sound of an electrical bell In that case we shall find 

 that, although the signal will come in exactly the same 

 way, practically the time occupied will be very consider- 

 ably longer, showing that a signal received by sound takes 

 longer in producing its effect than one in which the signal 

 is felt by the tongue. 



In order to make all this perfectly plain I shall hand 

 round this tracing. You will see there several experiments 

 made with expected and unexpected signals, which show 

 the different results obtained in the two cases. 



The next question which arises, and with that I must 

 conclude what I have to say just now, is this : — You will 

 readily see that the exact measurement of time depends 

 upon the rate at which this clockwork happens to be 

 going. I happen to know that it makes twenty revolu- 

 tions per second. But suppose I do not know that. In 

 fact one would not trust to the accuracy of clockwork for 

 such a purpose. How should I then be able to measure 

 the duration of time so exceedingly short as the one which 

 now concerns us? In order todo this we always come 

 back to a physical standard, to a standard of absolute 

 invariability which we can depend upon as being true. 

 For this purpose we use a tuning-fork which produces 

 vibrations, the rate of which we know, because we know 

 the tone which the tuning-fork produces, and the arrange- 

 ment which is always used for this purpose is the one 

 shown here. We have turned off the voltaic current we 

 used for signalling, and turned it on the tuning-fork. 

 There are two electro-magnets on either side of the 

 tuning-fork which react upon it, so that the moment you 

 close the current the fork is thrown into vibration and 



produces its own characteristic note. All that we have to 

 do is, during the time we are making our record, to bring 

 this tuning-fork, which is now in vibration, into such a 

 position that this little brass pointer shall make a tracing 

 against the paper. If you look at the tracing I have sent 

 round you will find there are tracings on it of a fork, which 

 vibrates at the rate of 100 per second, consequently )ou 

 have nothing to do but to translate the tracings which 

 you have made and which correspond to the duration of 

 the mental process which you have been investigating, 

 into vibrations of the tuning-fork, and you get an exact 

 measurement of the total duration of the process. While 

 I have been doing this you hear the tuning-fork is in 

 vibration, and Mr. Page has made the tracings. After it 

 is varnished it will be sent round and you will see the 

 tracing made by the fork over the traces corresponding to 

 the different experiments we made just now. 



I may observe that although the experiments made on 

 that paper were made with myself, you find that the 

 period occupied by the reflex is considerably longer than 

 in the other which I sent round previously. But that 

 one may very easily explain from the abnormal conditions 

 under which the experiment has been made as regards 

 myself. 



1 intended to go on from this subject to another 

 mode of investigation, namely, to the very beautiful in- 

 struments which have been lately introduced for the 

 purpose of measuring the finest differences of bulk in 

 different organs, as for example, in the human arm, by 

 which you can ascertain the condition of the circulation 

 precisely by a very exact registering- measurement of the 

 bulk of the arm;^ but as there are several other gentlemen 

 now ready to address you, I will defer that till this after- 

 noon. I will now conclude what I have to say by asking 

 you to listen to Dr. Hooker. 



SCIENCE IN GERMANY 

 {From a German Correspondent.) 



HERR V. OBERMAYER has recently communicated 

 a memoir to the Vienna Academy on the re- 

 lation of the coefficient of internal friction of gases to 

 the temperature. If we accept for the coefficients of fric- 

 tion ft at ^° C, the formula — 



where a is the coefficient of expansion of the gas, taken 



as basis of the calculation, then the experiments of Ober- 



mayer give the following results : — 



For Air 



,, Hydrogen ... 



„ Oxygen 



,, Carbonic oxide 



,, Ethylene ... 



,, Nitrogen 



,, Protoxide of nitrogen 



,, Carbonic acid 



,, Ethyl chloride 

 The coefficient of friction of the permanent gases is, 

 according to these experiments, approximately propor- 

 tional to the J-power of that of the coercible gases, and to 

 the I -power of the absolute temperature. 



For temperatures between 150° and 300° C, air gave 

 the same values of n as between the lower temperatures 

 - 2i°-s and 53°-5. In the case of carbonic acid a slow 

 decrease of the exponent n with the temperature was 

 perceptible from the experiments. W. 



n = 076 

 n = 070 

 n = o-8o 

 « - 074 

 « = 096 

 n — 074 

 « = 093 

 n = 0-94 

 n = oq8 



NOTES 



On Tuesday a visit was paid to the Challenger at Sheemess 

 by several Fellows of the Royal Society, foreign men of Science, 

 who are in London in connection with the Loan Collection 

 « The apparatus was fully described subsequently by Mr. Gaskell. 



