478 



NATURE 



[March i6, 189^ 



J ought (in the absence of experimental errors) to be identical 

 at all temperatures. The close agreement between the values 

 from different groups, and from the same group at different 

 temperatures, is a satisfactory proof of the accuracy of our 

 determination of the water equivalents of the calorimeter, and 

 of the changes in it and in the capacity for heat of the water. 



Hence, if we assume 



I- The unit of resistance as defined in the "B.A. Report," 

 1892; 



2. That the E.M.F. of the Cavendish Standard Clark cell at 

 15° C. = I "4342 volts ; i 



3. That the thermal unit = quantity of heat required to raise 

 I gram of water through i° C. at 15° C., 



the most probable value of 



J = 4'1940 X 10^.- 



This, by reduction, gives the following : — 



J = 427*45 kilogramme-metres in latitude of Greenwich 

 (^= 981 -I?)- 



J = 1402 "2 foot-pounds per thermal unit C in latitude of 

 Greenwich (^ = 32"i95). 



J — 778*99 foot-pounds per thermal unit F in latitude of 

 Greenwich {g = 32*i95). 



The length of this abstract is already unduly great, and we 

 will, therefore, not enter on any discussion of the results beyond 

 remarking that if we express Rowland's value of J in terms of 

 our thermal unit we exceed his value by i part in 930, and we 

 exceed the mean of Joule's determination by i part in 350.^ 



The difference between Rowland's value of the temperature 

 coefficient of the specific heat of water and ours would, however, 

 cause both his and our values of J to be identical if expressed in 

 terms of athermal unit at 1 1 "5° C. 



March 2. — "The Effects of Mechanical Stress on the Elec- 

 trical Resistance of Metals." By James H. Gray, M.A., B.Sc, 

 and James B. Henderson, B.Sc, International Exhibition 

 Scholars, Glasgow University. Communicated by Lord Kelvin, 

 T.R.S. 



This investigation was begun for the purpose of obtaining an 

 easily worked method of testing the effect of any mechanical 

 treatment on the density and specific resistance of metals. 



For alteration of density, copper, lead, and manganese 

 copper wires were tested. The effect of stretching was always 

 to diminish the density, the alteration being small however : for 

 copper about \ per cent., and for lead | per cent. The effect 

 of drawing through holes in a steel plate was somewhat greater, 

 showing at first an increase of 2 per cent. ; and, when the draw- 

 ing was continued, the density began to diminish till, after 

 drawing from diameter 2 mm. to i'3 mm., it showed an increase 

 on its original value of ^t per cent. Several other interesting 

 results on alteration of density were obtained. 



The most important part of the investigation, however, 

 relates to the alteration of specific resistance of copper, iron, 

 and steel wire due to stretching ; and, in connection with this, 

 the authors wish particularly to emphasise the advantages to 

 be gained from using the unit of specific resistance introduced 

 by Weber, who always defined it in weight measure, that is, as 

 the resistance of a length of the metal numerically equal to its 

 density and section unity. 



The conclusions arrived at are that for practical purposes any 

 mechanical treatment, however severe, does not affect the 

 electrical properties of the metals tested. As contrasted with 

 this, it is interesting to note that the smallest impurity in the 

 metal produces a greater change than the most severe me- 

 chanical treatment. For example, an impurity of f per cent, 

 lowers the electrical conductivity by 13-5 per cent., while an 

 impurity of* per cent, lowers it as much as 30 per cent. 



" A New Hypothesis concerning Vision." By John Berry 

 Haycraft, M.D., D.Sc. Communicated by E. A. Schafer,F.R.S. 



The author pointed out that when a blue pigment is mixed 

 with its complementary pigment — orange-yellow — it makes a 

 grey, not a green as is generally stated. This can be shown by 

 the use of transparent colours, such as watery solutions of the 



1 If we assume the E.M.F. of our Clark cells to be the same as that of the 

 •Cavendish standard (and we are inclined to think we have over-estimated 



the difference), we get J =4 '1930 X lo^. 



2 The value obtained by us in 1891 = (4*192 -1-) X lo^. 



3 Rowland obtained the mean value of Joule's determinations by assign- 

 ing values to different experiments, and the above comparison refers to the 

 numbers thus obtained. If, however, we attach equal weight to all Joule's 

 results, as reduced by Rowland, the mean exceeds our value by i in 4280, 

 --g our expression for the temperate "" ' ' ' "" ' 



NO. 1220, VOL. 47] 



aniline dyes. When you mix an opaque oil blue with its com- 

 plementary orange-yellew and get a green it is because the light 

 only passes through a very thin superficial film of the mixture, 

 and a paint which is orange-yellow in the mass is only a pale 

 yellow in a thin film, and transmits the green spectral rays 

 stopped by the orange-yellow. In this case, therefore, the thin 

 film of paint which alone affects the light is not a mixture of 

 blue and its complementary orange- yellow, but only a mixture 

 of blue and pale yellow. 



In the case of Maxwell's colour discs you get a grey 

 if the blue and yellow are complementary, or a green or 

 red if they are not, just as in the case of mixtures of 

 transparent pigments. Complementary pigments are simply 

 those which between them absorb all spectral rays ; thus blue 

 absorbs red, yellow, and some green, and the complementary 

 orange-yellow absorbs violet, blue, and some green. A mixture 

 of these pigments on the palette — if transparent enough — or on 

 the Maxwell's disc absorbs, therefore, the light which falls upon 

 it from all parts of the spectrum in about equal proportions. If 

 examined by the spectroscope the mixture of pigments and the 

 rotating disc both give a dim, unbroken spectrum identical with 

 that of white paper held in half light. In our study of vision 

 we have to deal with the stimulus — spectral rays — and the re- 

 sulting sensations. Inasmuch as the stimulus— the light of a 

 dim, unbroken spectrum — is the same whether the eye looks at 

 a mixture of blue and orange yellow on a palette, at a Maxwell's 

 disc, or again at a piece of white paper held in half light, the re- 

 sulting sensation must in all cases be the same — we call it grey or 

 white. In the case of the rotating Maxwell's disc experiment we are 

 not dealing with the fusion of blue and orange-yellow sensations, 

 but the adding together of two halves of the spectrum to make 

 a whole one. C)nce understood, the physiologist will discard 

 the experiment altogether, as it has no bearing upon colour 

 vision. 



The work of Sprengel, Darwin, and especially of Sir John 

 Lubbock, shows that the colour sense has gradually been evolved 

 by the coloured environment of the species. We may infer that 

 in the ancestral condition in which light was distinguished from 

 darkness, but blue was undistinguishable say from red, all visual 

 stimuli were felt as white or various shades of grey. The greater 

 the amount of spectral light the nearer the sensation approached 

 white. This, if accepted, explains why the outer and less used 

 parts of the retina are colour blind in the human eye at the pre- 

 sent day, and further explains why a minimal stimulus from a 

 coloured object gives rise to a sensation grey. Just as we may 

 smell something, but require to "sniff," in order to make out 

 what it is, so the coloured object held far away may give rise 

 only to the primitive sensation grey, and has to be brought nearer 

 in order that its colour quality can be felt. 



We may explain the fact that an artificial mixture of spectral 

 green and red gives rise to the sensation yellow by the fact that all 

 coloured objects which send to the eye red and green rays also 

 send the intermediate yelLaw ; these objects give rise to the sensa- 

 tion yellow, and we call them by that name. Inasmuch as this 

 association of red and green rays has in the evolution of the eye 

 always combined with yellow rays to produce the sensation 

 yellow, we can explain, as an instance of association, the fact 

 that artificially combined red and green rays produce a yellow 

 sensation. 



When, say, red and blue-green spectral rays are artificially 

 combined, they produce a grey sensation, and this we can 

 explain by the fact that no fully coloured natural object sends 

 to the eye such a combination, which combination, therefore, 

 played no part in the evolution of the colour sense, and it 

 produces merely a primitive sensation of simple brightness — white 

 or grey. 



That a coloured object brightly illuminated appears white 

 follows the law of maximal stimulation, for in this case 

 the object absorbs so slight a proportion of the light from any 

 one part of the spectrum that that part gives rise to its maximal 

 effect, and the rest of the spectrum can do no more. In this 

 case, therefore, the eye is affected equally (maximally) by all parts 

 of the spectrum, and we have of course the sensation of white. 



The above view is an attempt to explain some of the facts of 

 vision by showing that they are on all fours with other facts 

 known to the physiologist. This seems to the author a more 

 scientific method than the one adopted by Young and Helmholtz, 

 who "conceive "a visual apparatus, and endow it with such 

 properties as will, in their opinion, account for the factsof visual 

 sensation. 



