3S 



NA TURE 



[November 14, 1907 



law that the observation of a motion which is being 

 stopped is in favourable circumstances followed by the per- 

 ception of that motion in reversed direction. 



The accompanying figure shows the card with its plane 

 passing through the observer o. L and r are the inter- 

 sections of the circumference of the card with an arbitrary 

 horizontal line. Suppose that the card turns round its 

 axis PQ from north to west, the observer will see l 

 removing to the left and R to the right, and from this 

 he can conclude the e.xact direction of the rotation, pro- 

 vided that he makes sure of L being behind and r before 

 ro. and not otherwise. For suppose l before pq in l' 

 and R behind pq in r' the observed removal of these 



points respectively to the left and to the right would lead 

 to an apparent opposite direction of rotation — from north 

 to east. Moreover, the axis pq perpendicular to lr would 

 seem to tilt away from the vertical to p'q' perpendicular 

 to l'r'. Hence the illusion depends upon the following 

 condition : — if L and r are seen in the right place with 

 respect to each other, the rotation appears in its true 

 direction, but if, on the contrary, l and r are seen in 

 l' and r', so in the wrong place with respect to each 

 other, the card will seem immediately to reverse its dircc- 

 lion of rotation, and the axis will seem to tilt away from 

 the vertical. 



As an observer, viewing the true direction of the rotating 

 card, will generally be unable to distinguish the right place 

 of the two points when he shuts one of his eyes, the 

 circumstances are favourable for the reversing of the direc- 

 tion of the rotating card. 



Utrecht, October 5. L. U. H. C. Werndly. 



The Interpretation of Mendelian Phenomena. 



If I have read Dr. Archdall Reid's letter in Naturf. of 

 October 31 aright, he draws a distinction between the 

 study of heredity in general and the study of the problems 

 of sexual reproduction, now defined as the problem of the 

 function of se.x (an expression with which I am perfectly 

 contented). Among the problems of sex he includes the 

 study of the actual transmission of characters as dealt with 

 by Mendelians. The novelty of this classification is 

 certainly attractive ; but I find it difficult to understand 

 what branches of knowledge remain to fall under the 

 former head. In what does the* study of heredity consist 

 if not in the study of the transmission of characters from 

 parents to offspring? 



If by this apparent paradox it is only meant to imply 

 that the Mendelians must confine their study to the trans- 

 mission of characters by the sexual method, they may seek 

 comfort in the reflection that this is by far the most 

 important of all branches of heredity — it is the only one, 

 for example, which affects the human race. Indeed, the 

 rule that all organisms pass through a sexual cycle at 

 some period of their existence has extremely few excep- 

 tions ; but I, for one, see no reason for restricting the 

 experimental study of heredity even to this extent. Mendel 

 demonstrated the segregation of the germinal represent- 

 atives of certain characters in the reproductive cells. What 

 reason is there for doubting that such segregation may 

 take place among the ova of a parthenogenetic individual? 

 There is, in fact, evidence of the actual occurrence of such 

 segregation. More than this, we know of segregation 

 where reproduction is purely vegetative, as in the case of 

 bud sports. 



It is the claim of the Mendelians that they have dis- 



NO. 1985, VOL. ']■]'] 



covered in certain cases some of the fundamental 

 characters of an organism — the units of hereditary trans- 

 mission, which are represented in the reproductive cells by 

 definite entities known as allelomorphs. Sometimes these 

 characters are identical with those which can be defined 

 by simple inspection previous to experimental analysis, 

 sometimes they are not. Sometimes the apparent character 

 depends upon the simultaneous presence of several allelo- 

 morphs, each of which may segregate from its opposite 

 in complete independence of all the others. Mendel him- 

 self suggested that this conception, the proof of which he 

 left to his successors, might afford the explanation of 

 rertain botanical cases which do not appear to be widely 

 ilifl'erent from that of the mulatto. If Dr. .'\rchdall Reid 

 will produce authenticated pedigrees showing the repeated 

 crossing of the mulatto with pure white blood and pure 

 black blood respectively, together with a detailed account 

 of all the offspring produced, he will make a very sub- 

 stantial contribution to our knowledge of heredity in the 

 human race, and one which will be examined with very 

 great interest by Mendelians. In the absence of such 

 evidence the statement that there is no segregation does 

 not seem to me to be justified, even in this particular 

 instance. 



It has been pointed out that Mendel's discovery is lead- 

 ing to a change in our conception of the constitution of 

 an organism comparable with the change which the advent 

 of the atomic theory produced in chemists' conceptions of 

 compound substances. Whatever biological problem we 

 may now discuss, Mendel's facts have to be reckoned with. 

 It is true that the only method so far discovered of study- 

 ing the constituent characters of organisms consists in the 

 crossing together of individuals in which some of the 

 characters are different. This method is so closely com- 

 parable with that by which the chemist studies his com- 

 pounds that Mendelians have often found a readier 

 appreciation of their views among students of the more 

 precise physical sciences than among biologists. The 

 advantage of introducing exact experiment into the study 

 of heredity ought to be obvious to all, and I fail to see 

 any other objection to the method except its novelty. 



if Dr. Archdall Reid desires to grasp this new concep- 

 tion, I can only recommend him to a renewed study of 

 the literature of the subject, beginning with Mendel's own 

 papers. Better still, let him repeat a few of the simpler 

 experiments. There is no royal road to this knowledge ; 

 but it is knowledge which is rapidly revolutionising our 

 entire conception of the constitution of a living organism. 

 Dr. Archdall Reid is so far from appreciating this at 

 present that further discussion seems likely to be of very 

 little profit. I will conclude my contribution to this con- 

 troversy with a word of warning. If Dr. Archdall Reid 

 discards Darwin's opinion, based as it was upon an un- 

 equalled experience, that domestic and natural varieties 

 have arisen by essentially the same process, he may find 

 himself landed among a crowd of unsuspected difficulties. 



Cambridge, November 2. R. II. Lock. 



Method of Observing the " Subjective Yellow." 



A SIMPLE method of obtaining the sensation of yellow 

 produced by the mixture of red and green lights is afforded 

 by a small direct-vision spectroscope of the ordinary kind 

 in which the slit can be rotated to adjust its line per- 

 pendicular to the plane of refraction. If the slit is turned 

 slowly from this normal position, the bands of different 

 colours of course take up a sloping direction across the 

 spectrum, like books on a half-filled shelf. .As the slope in- 

 creases, the upper end, for example of the red, closes down 

 on the lower end of the green, and as the two blend the 

 clear yellow tint is produced. Other colour mixtures can 

 be similarlv noticed. 



It may be added that if the slit is turned thus until 

 its length lies in the plane of refraction, the violet end 

 of the impure spectrum obtained is drawn out and so 

 more easily observed than in the normal method of use, 

 and is still pure enough for most of the purposes for 

 which a simple spectroscope is of use. 



John H. Sii.\xhy. 



University College, Cardiff. 



