August 17, 1893] 



NA TURE 



381 



black and white card moved horizontally in front of the eyes. 

 The estimates tha'. I have made with proper apparatus very 

 closely correspond with the value as originally determined by 

 Charpentier. With a disc revolving once in two seconds, I fi id 

 the apparent angu'.ar magnitudes of the two phases equal to 

 about 2° '5 ; with a disc revolving twice as fast they are about 5°. 



(2) Charpentier' s experiment of the Fluted Band is somewhat 

 more difScuIt of performance and of interpretation. A black 

 disc, 45 cm. in diameter, revolving about twice per second, 

 with a small white spot (l cm. X 'S), 20 cm. from the centre. 

 Observer's eye fixed upon a bead placed in front of the disc at 

 that distance from the centre. Under these circumstances the 

 white spot appears stretched out to a white band with indetinite 

 beginning and end, which appears to be composed of several 

 alternately lighter and darker portions of longer light internodes 

 with shorter dark nodes. Whereas in the experiment of the 

 black sector, the apparent angular magnitude increases with in- 

 creased speed of revolution, in this experiment the angnlar 

 magnitudes of the nodes and internodes diminishes with in- 

 creased speed (or what amounts to the same thing, with 

 approximation of the observer's eye to the disc) and vice-vers&. 



Charpentier explains this at first sight very puzzling relation 

 by the following hypothesis, which is at the same time an in- 

 genious application of a well-known physical principle to a 

 hypothetical physiological wave transmission and a proof of the 

 existence of the latter. Upon the incidence of the stimulus 

 white, an oscillation of sensation is produced, of which the first or 

 positive phase is white, the second or negative phase black ; each 

 phase has a duration of abouto 015 sec. — i.e. the entire oscilla- 

 tion has a duration /=0'03 sec. and a frequency «cf 33 per sec. 

 This much is demonstrated by the experiment ol the black sector. 

 Let us now suppose that the? oscillation spreads from its origin 

 in the organ of vision ' over the remainder of that organ, as an 

 oscillation at one point of a pond spreads over the remainder 

 of the pond. The problem is to determine the velocity of 

 transmission v and the wave length / of this presumably 

 propagated oscillation. This is done by Charpentier by the 

 following physiological application of Doppler's principle re the 

 apparent modification of sound-waves according as the distance 

 between origin and ear is increasing or diminishing. 



In accordance «yth a familiar relation, wave-length / is equal 

 to velocity v, multiplied by duration /, or / = vt. In accord- 

 ance with Doppler's principle the apparent rise of tone or the 

 apparent diminution of wave-length caused by the approxima- 

 tion of observer and wave origin, are such that I' = (v - v')t, 

 where /' is the apparent wave-length, and v' the velocity of 

 approximation. 



If we were debarred from measuring tones proceeding from 

 stationary origins, we might nevertheless determine their wave- 

 length and velocity by calculation from measurements of the 

 apparent wave-lengths of tones proceeding from origins moving 

 ,it different known velocities. From two equations, i' = {v-v')t. 



r = (v ~ v")l, we should have t = 



/' - I" 



e - 



■ V . I 



—- and (substituting ^ 



t V 

 - rv' 



for t) V 



- and its reciprocal, 



I'v" - l"v' 



I' - /" 



and 



v" - v' 



These are, in fact, the data experimentally accessible in the 

 retinal phenomenon. We cannot (as far as i.s known at present) 

 measure the velocity an 1 length of a retinal wave with stationary 

 origin ; we must determine these values from their apparent 

 values with the wave-origin moving at different known veloci- 

 ties. 



Practically the velocities v', v", &c., of the wave origin on 

 the retina are easily adjusted ; the apparent wave-lengths /', /", 

 lie , more or less accurately observed. Given the di.-nensions 

 of the disc, its distance from the eye and its rate of revolution, 

 the experimental velocities are easily calculated ; similarly if 

 the apparent dimensions on the disc of the nodes and internodes 

 ,ire accurately observed, the retinal wave-lengths corresponding 

 with them can be accurately calculated. It is in this second de- 

 termination that the chief experimental error can arise ; never- 

 Itheles?, considering the original conditions of the problem and 

 1 .and that this is, in fact, the first time it has been approached 



I, * It is essentially in iifferent whether we take or.;an of vision to signify 

 f [the retina or brain or re'.ino-cercbral apparatus. It is convenien: to refer 

 ' I measurements to the retina itself, aid to determine retinal veloc'.ty and 

 , ' retinal wave-length. 



NO. 1242, VOL. 48] 



and solved by any method, the results given by Charpentier are, 

 within limit.=, sufficiently demonstrative of the propagation of a 

 retinal oscillation and of its approximate velocity and wave- 

 length. He finds from a large number of measurements a 

 velocity between the limits of 53 8 and 90 mm. per sec. (mean 

 value, 72); a frequency between 28 and 54 (mean value, 36) ; 

 a calculated wave-length on the retina of 2 mm. ; and a calcu- 

 lated wave-duration of o 028 sec. 



Not the least satisfactory feature of these figures is that the 

 value of the wave duration derived by the indirect method of 

 this more difficult experiment, practically coincides with that 

 derived from the simple and easy experiment of the black 

 sector. 



A third experiment of Charpentier's, although not precisely 

 confirmatory of these, seems to stand in some relation to the 

 negative semi-vibration manifested as the black sector. A black 

 disc with open sectors, revolving 'uetween the eye and a white 

 sheet illuminated by direct sunlight, gives rise to the sensation 

 of a magnificent purple colour, when the rate of revolution is 

 such that the eye receives between 40 and 60 stimuli per second, 

 i.e. when each stimulus occurs during the negative phase of 

 the preceding stimulus. Above 70 and below 30 stimuli per 

 second the sheet appears white. The effect is very striking and 

 very easily obtained ; in short, it is a " ladies' e.xperiment " ; its 

 full explanation is a different matter, and far too uncertain for 

 discussion in a short article. A. D. W. 



THE POSITION OF SCIENTIFIC EXPERTS. 



1.7 ROM time to time it has been pointed out in these columns 

 that the services rendered to litigators as such by so- 

 called scientifie experts is antagonistic to the pure spirit 

 that should actuate men of science. For some years the 

 position and character of the representative of science in 

 courts of justice has been acquiring interest, not only in 

 England but elsewhere. In faci, a few years ago a Committee 

 of the American Association for the Advancement of Science 

 was appointed to consider the whole matter, but no report of 

 their proceedings has yet been published. An excellent dis- 

 cussion of the subject, however, comes from America in the 

 form of a reprinted lecture on " The Scientific Expert in Foreign 

 Procedure," by Prof. C. F. liimes, which appears in the June 

 number of the fournal of the Franklin Institute, In order to 

 direct the discussion. Prof. Himes first gives legal opinions as to 

 the status of the expert. "Justice Miller," he says, "ex- 

 hibited a plan of objection in a charge as follows : — ' My own 

 experience, both in local courts and in the Supreme Court of 

 the United States is, that when the matter in contest involves 

 an immense sum in value, there is no difficulty in introducing 

 any amount of expert testimony on either side. ' Another judge, 

 in a lecture upon medical expertism, gives a similar opinion, 

 that the ground of dissatisfaction in regard to medical testimony 

 to both the professions of law and medicine, are reducible to 

 one — that upon every conceivable issue expert opinions are 

 procurable which sustain, or seem to sustain, the most contra- 

 dictory views." But Prof, tlinies does not take a pessimistic 

 view of the scientific expert. He is inclined to believe that : — 



" The scientific expert is simply a product, and an extreme 

 product, of an advanced and rapidly advancing civilisation. 

 He was recognised in the germ, to be sure, by the old Roman 

 law, and we may assume in all systems of jurisprudence ; but he 

 has acquired an immensely increased importance, and a much 

 wider field and a far greater frequency of employment by the 

 recent, and very recent, marvellous advances in the applications 

 of science— applications which have increased the sphere of 

 things to be litigated about, which have introduced facts of an 

 entirely new character to be adjudicated upon, to say nothing 

 of the contribution that science has made, and is continually 

 making, in many ordinary cases, of conclusive missing links of 

 evidence which render decision previously uncertain, comfort- 

 ably certain, and satisfactory. 



' 'Now, one fact that seems latent in these expressions of the 

 legal profession in regard to the scientific expert and almost the 

 first that impresses is that in many respects he seems to be a 

 positive annoyance to lawyers, and even to judges at times — a 

 sort of intractable, incompatible, inharmonious factor, disturb- 

 ing the otherwise smooth current of legal procedure ; too 

 important or necessary to be ruled out, too intelligent and 

 disciplined mentally to yield without reason to ordinary rules 



