ii8 



NATURE 



\yune I, 1876 



cube of one-fifth of a millimetre in measurement the 

 enumeration would be somewhat easier, but still impos- 

 sible, for the number contained in such a cube would be 

 enormous ; and therefore, it is necessary to diminish the 

 bulk of the blood in which you make your counting very 

 much further. This j ou can only effect by a process of 

 dilution. In order to get at your result you have not 

 only to diminish the bulk of the quantity which you con- 

 template and in which you count, as much as possible, 

 but also to dilute the blood so that your liquid may contain 

 a very much smaller proportion of blood corpuscles. 

 You dilute it then 250 times, and in this way you divide 

 the cube of a miUimetre from which you started, into 

 about 31,000 parts, and count the blood corpuscles in the 

 thirty-one thousandth part of a cubic millimetre. Sup- 

 posing you find it contains about 160 corpuscles you will 

 find by calculation that they amount to about 5,000,000 

 in the whole cube from which you started. This being 

 the case the question is how we effect the division. We 

 do it in this way : You first dilute your blood in the exact 

 proportion required, and for this purpose one uses the 

 apparatus which is on the table. You take a capillary 

 pipette which will only take an extremely small quantity, 

 in fact, a cubic millimetre of blood. Then having filled 

 your pipette you discharge it into a little eprouvette, into 

 which has been introduced 250 times, or rather 249 times, 

 the bulk of some liquid with which blood can be diluted 

 without its corpuscles being destroyed. Having thus got 

 this diluted liquid which contains blood in the proportion 

 I have mentioned, all that you have to do is to place 

 under the microscope a layer of a definite thickness — 

 one-fifth of a millimetre — and count the number of cor- 

 puscles in a square of the same measurement. That is 

 effected by this very ingenious arrangement, which was 

 introduced by M. Potain, and has been finally perfected 

 by Messrs. Hayem and Nachet. The way it is done is 

 this : An object-glass is covered by a perforated plate ; 

 the perforated plate is of the thickness I mentioned, 

 namely, exactly one-fifth of a millimetre. Consequently 

 if a very small drop of the mixture of the blood with 

 serum (the diluting hquid) is placed within this space, 

 you have a layer of the thickness I have mentioned which 

 you can contemplate. You can cut off a cubic millimetre 

 of that stratum of blood perfectly easily by means of a 

 micrometer eyepiece, and in that way accomplish the re- 

 quired enumeration. You have in short before you a 

 quantity of liquid which contains about the thirty-one 

 thousandth of a cubic millimetre of blood, and conse- 

 quently would obtain, if the blood were normal, 160 

 corpuscles. These can be very readily counted, and the 

 whole process can be done in a very few minutes — in a 

 much shorter time, in fact, than I have taken to describe 

 it to you, and you get results which are not only equal 

 to those obtained by chemical investigation, but more 

 accurate. This, I think, is a good example of the appli- 

 cation of the microscope as an instrument of measure- 

 ment to an important question. 



The next subject that I wish to draw your attention 

 to is a dififerent one. It is a question of measuring the 

 time occupied in certain simple processes in which the 

 nervous system is concerned. The examples I am going 

 to give you are entirely derived from the physiology of 

 man, and relate to the phenomena which we observe in 

 ourstlves. The measurement to which I wish to draw 

 your attention is the measurement ot the time occupied in 

 what we call in physiology a " reflex " process. You may 

 reasonably ask that I should endeavour to explain what a 

 reflex process is, and the only way, or at any rate the 

 readiest way in which I can do this is by giving you an 

 example. Supposing this blank card, which has written 

 on it previously some word, say the word " reflex," were 

 suddenly turned over by a second person. It is agreed 

 that at the moment I see the word upon it, I say the word 

 " reflex." In that act it is obvious that there are three 



stages. First, the reception of the impression by my eye 

 produced by seeing the word ; secondly, the process which 

 goes on in my brain in consequence of seeing it ; and 

 thirdly, a message sent out from my brain to the muscles 

 which are concerned in articulation, by means of which 

 certain movements are produced which give rise to the 

 sound which you recognise as the word " reflex." That is 

 one example. Let us now take another which is simpler. 

 We cannot take one better than the act of sneezing. Some 

 snuff finds its way into the nose ; an impression is received, 

 a change is produced in one's nervous centres, and in 

 consequence of that central change, a certain number of 

 muscles are thrown into the action recognised as sneezing. 

 These are different examples of reflex action. The brain, 

 the highest part of the nervous system, has to do with the 

 first ; whilst the other is one in which the nervous centres 

 lower down have to do, and consequently it is simpler. 

 The methods which I am going to illustrate to you are 

 methods intended for the measurement of the time occu- 

 pied in this process. First, let me draw your attention to 

 the circumstance that you have here three stages. You 

 have the stage of reception ; the stage corresponding to 

 the changes which take place in the brain in consequence 

 of the reception of an impression from outside ; and 

 thirdly, the process by which you convey the effect 

 to the muscles which act. Now let us agree, in 

 speaking of this, to call the impression the " signal," 

 and to call the muscular effect the "event." In 

 that case the question before us is to measure how 

 much time takes place between the reception of this 

 signal by a certain person and the occurrence of the 

 event, namely, the completion of the muscular action. 

 There are a great many questions involved in this : thus 

 you may measure either the whole process or one of its 

 stages. You may measure, for example, either the time 

 occupied by the reception, the time occupied by the dis- 

 charge, or, on the other hand, the time which is occupied 

 by the changes which take place in the centre itself. In 

 the first instance I gave you just now — the example of 

 reading a word aloud — the time occupied in the reception 

 is extremely short, and the time occupied in the discharge 

 is also extremely short. Popularly the whole thing is 

 done as quick as thought, but, comparatively, the time 

 the brain takes in going through these changes which 

 connect the reception of the impression with the discharge 

 is a very considerable one. AH this we can make out 

 with absolute accuracy by methods of measurement. 

 Most of these methods are founded on this principle, 

 that we measure the duration of a voltaic current which 

 is closed at the moment the signal is given, and opened or 

 broken at the moment that the act takes place. There 

 are a great many instruments conL.tructed on this prin- 

 ciple, of which you will find illustrations in the next room. 

 The general principle involved in all of them is shown 

 on this diagram. In the simplest form you can give to 

 such an apparatus you must have a surface of paper so 

 placed that it shall pass horizontally by the point of the 

 lever, and at a uniform rate j thus, for example, it may 

 pass at the rate of i metre in the second. Supposing 

 this to be the case, it is obvious that if you arrange the 

 electro-magnet so that when you close the current a 

 certain mark is made, and that at the moment of the 

 break of the current when the magnet ceases to act, another 

 mark is made, you will have a tracing on the surface of 

 the paper which indicates the time. So long as nothing 

 is going on, the paper receives a horizontal mark, but at 

 the moment the signal is given you have the point of the 

 lever descending. At the moment the act takes place the 

 lever assumes its original situation, and you have again a 

 horizontal line. That is the general principle of the 

 apparatus. Now for its action. We have here a voltaic 

 circuit and a key by which we can give the signal. I 

 shall be the subject of the experiment, and you will 

 see what the result is. Here is the recording arrange- 



