70 



NA TURE 



[March 17, 1910 



elusive by all the examiners, it is most desirable that some 

 method should be used about which there could be no 

 doubt whatever. Such a method, 1 believe, is one founded 

 on the exact measurement of the persistence of vision of 

 colours througliout the spectrum, when the frequency of 

 flicker is at its critical value. The only thing the person 

 under examination has to do is to adjust the speed of a 

 sectored disc so that the flickering of the part of the spec- 

 trum under observation just ceases. The speed of the disc 

 may be automatically registered on a chronograph by 

 electrical means. A series of measurements throughout 

 the spectrum when plotted gives a " persistency curve," 

 which can readily be compared with a similar curve for 

 a normal eye. The great advantage of this test is that 

 it is purely mechanical, and does not involve the handling 

 of coloured materials or the judging of colours in any way 

 by the candidate. 



A careful study by this method of twenty-six cases of 

 colour-blindness which I made some years ago (" Persist- 

 ence of Vision in Colour-blind Subjects," Phys. Rev., 

 vol. XV., 1902) showed such systematic differences between 

 eyes with normal and those with abnormal colour-vision 

 that I am convinced it is the most exact method yet used 

 for the study, not only of colour-blindness, but also of 

 normal colour-perception. I can the more readily express 

 such an opinion inasmuch as this method, though used 

 very extensively by myself, was originally devised by Prof. 

 E. L. Nichols. 



In the article in Nature there is the statement that in 

 the study of colour-blindness " there is none of that 

 accuracy of definition in the scientific picture which 

 rejoices the heart of the physicist." To some extent this 

 may, perhaps, be true, especially for very strong or weak 

 intensities of stimulus. On the basis of the cases I have 

 referred to above, I venture to assert that, for ordinary 

 daylight, there is far more " accuracy of definition " than 

 is usually supposed to exist. I do not know how many 

 cases of colour-blindness must be studied to render the 

 conclusions drawn therefrom reasonably sure, but those to 

 which I refer can readily be divided into distinct classes 

 on the basis of three fundamental colour-sensations, red, 

 green, and violet. 



Holmgren, I believe, first pointed out the possibility of 

 the following varieties of colour-blindness : — 



Red-blindness ^ t~w /■ .• • 1 



Green-blindness [De ective m one colour sensa- 



Green-blindness 

 Violet-blindness 



tion. 



Red-green blindness ...Ir-k /■ .^' • ^ 1 



Red-violet blindness ... IDefective in /zvo colour sensa- 



, Green- violet blindness ...J "°"^- 



Red-green-violet hj^ndness\ Defective in i/iree colour sensa- 

 or total colour-bliijbdness/ tions. 



In the paper on colour-blindness referred to above, there 

 is, I believe, a demonstration of the existence of each of 

 the above types with the single exception of violet-blind- 

 ness. This classification, which was arrived at by means 

 of persistency curves, seems to me to be as definite as 

 could almost be desired. 



I would therefore suggest this method for the examina- 

 tion of at least doubtful cases of defective colour-vision. 

 The only objection is that the measurements, while simple 

 and easily performed, are somewhat tedious if the spectrum 

 is to be thoroughly examined. 



Frank Allen. 



Physics Department, University of Manitoba, 

 Winnipeg, February 11. 



Practice and Knowledge. 



Mr. and Mrs. Howard in Nature of February 17 show 

 that " the past history of agricultural science furnishes 

 several examples of belated explanations of the utility of 

 practices the value of which has long been a tradition 

 among practical men." In other departments of life prac- 

 tice in advance of knowledge is frequent, and there is one 

 which struck me recently, and may have been observed by 

 others, which is the practice of blowing hot and cold with 

 the mouth which ^sop makes use of in his fable of the 

 ."Satyr and the Traveller," and has given rise to the 

 common disparaging saying of " blowing hot and cold." 



Some may suppose that the whole explanation of blow- 

 NO. 2107, VOL. 83] 



ing hot and cold may be in the difference in temperature 

 of the two bodies blown on ; the Traveller's hands in 

 yEsop's fable were colder than his breath, while the heated 

 mulled wine was warmer. This, however, is only a partial 

 explanation. We have unconsciously acquired the practice 

 of blowing at different temperatures. If we wish to warm 

 our hands we open the mouth wide and direct a slow 

 moving current of air to them, or, more properly expressed, 

 we breathe on our hands. This current has nearly the 

 body temperature ; but when we wish to blow cold we 

 purse in the lips until there is only a small opening, as in 

 whistling, and discharge a fine jet of air under pressure. 

 This jet entangles a large amount of air with it, and 

 when it arrives at the hot surface its temperature is much 

 lower than that of the breath. Should the hot surface be 

 also moist, the current of air quickens the evaporation, 

 and so hastens the cooling. 



In passing, it may be noticed here that ^sop and the 

 modern use of the expression " blowing hot and cold " 

 seem to have missed the mark. The objects blown on are 

 not the same, but different, and require and receive 

 different treatment. It is no (fPs^irtigement to say of • a 

 man that he blew hot on a scheme which seemed to him 

 to require encouragement, while he blew cold on another 

 he thought ought to be suppressed. ^sop putting the 

 words into the mouth of a being of the type of a Satyr 

 seems to suggest he was not quite sure he had given the 

 highest interpretation of the incident recorded in the fable. 



John Aitken. 



Accelerated Velocity of Jupiter's Red Spot Hollow. 



The longitude of the middle of the Hollow has shown 

 a comparatively rapid diminution since the beginning of 

 the present apparition, as the figures below clearly 

 indicate : — 



Month 



Mean Diminu- 

 longitude tion 



Month 



ipog Oc'ober ... i5"3 

 1909 November... 13 3 

 1909 December... 11 "8 



— I igio January 

 2'o I igio February 

 '■5 i 



Mean Diminu- 

 longitude tion 



8°8 ... 3°o 

 7-4 ... 1-4 



From its estimated position on October 25 and 

 February 25 (the first and last dates of observation in the 

 above table), a rotation period of gh. 55m. 37-gs. (287 

 rotations) has been deduced. This is 2-7 seconds shorter 

 than the adopted period. 



If the present accelerated rate of motion continues until 

 June next, the centre of the Hollow will arrive at \ 0°, 

 or, rather, its longitude will coincide with the zero meridian 

 of system ii. 



Now this quicker velocity became evident towards the 

 end of the last apparition. An examination of my transit- 

 chart of the Red Spot region for that epoch shows that, up 

 to the beginning of May, 1909, the Hollow exhibited a 

 normal monthly increase in longitude of i-o°. Subsequently 

 it began to move decidedly quicker, and this acceleration 

 has been well maintained up to the present, as will thus 

 be seen : — 



Date X BUv?^d Rotation Remarks 



rotations period 

 „ h. m. s. 



1908 Dec. 20 ... i3"6) . fMay 2 chosen a? approximate date 



1909 May 2 ... i8'i> ^^' "■ ^ ^^ *' '( of change in velocity 



1909 May 2 ... i8*i\ . fThe last transit of the apparition 



1909 June 12 ... i8'4) ^' "" ' ^^ ^° 'l was taken on June 12 



1909 June 12 ... i8*4'i . /Planet invisible for greater part of 



1909 Oct. 25 ... is's/ ^^' ■■■ 9 55 40 S I tifne between the two dates 



1909 Oct. 25 ... is'3\ o . /Hollow in conjunction with .S. 



1910 Feb. 25 ... 7-o/ '^ '" ^ ^5 37 9^ Tropical Dark Area, since January 



The approximate date when the present accelerated 

 velocity first commenced may be regarded as May 2, 1909. 



The recent behaviour of the Hollow, or the Red Spot, is 

 attributed to one well-known phenomenon. Once about 

 every twenty-three months the Great South Tropical Dark 

 Area passes rapidly by the Spot, and on each occasion the 

 latter temporarily participates in its movements. As it is 

 now two years since we witnessed the last of these 

 periodical occurrences, the dark matter, having swept round 

 the planet, is once again involving the region of the Red 

 Spot. The recent accelerated motion of the Hollow, there- 

 fore, was fully anticipated. 



March 4. Scriven Bolton. 



