558 



NATURE 



[April i6, 1896 



they do not merely give the cards away, but themselves collect 

 them and forward to me, as my experience has so fully demon- 

 strated that not 5 per cent, of the recipients will take the trouble 

 of returning them directly to me. 



A collection of looo family measurements would be of im- 

 mense value for testing various problems in heredity, and, if it 

 be formed, I shall be only too glad that it should be ultimately 

 deposited where it would be available for all future students of 

 heredity. 



It is, perhaps, needless to remark that the measurements 

 required are of normal and not pathological characteristics, and 

 no name except that of the recorder (in case of there being need 

 of reference) is required. 



All communications should be addressed to me at Urjiversity 

 College, London. Karl Pearson. 



University College, London, April lo. 



The Retinal Circulation. 



The phenomenon of the retinal circulation, referred to in Mr. 

 James W. Barrett's letter in Nature of April 2, was, I believe, 

 first described by Dr. Robert Waring Darwin in 1786. He 

 writes: — "By being accustomed to observe such small 

 sensations in the eye, it is easy to see the circulation of the blood 

 in this organ." . . . " It is sometimes necessary to rub the eyes 

 with a certain degree of force after they are closed, and to hold 

 the breath rather longer than is agreeable, which by accumulat- 

 ing more blood in the eye, facilitates the experiment." He 

 further states that it depends on the state of health, is most 

 distinct when the eye is fatigued, and is best seen by looking at 

 the sky, shading the eyes with the hand. 



I have myself seen it often when a boy under much the same 

 conditions as your correspondent, viz. lying on my back looking 

 up at the blue sky afler severe exercise. 



But about fifteen years ago I discovered a method of observ- 

 ing it far more perfectly. While calibrating a somewhat power- 

 ful spectroscope by measuring the Fraunhofer lines with a very 

 narrow slit and direct sunlight, I noticed that when the region- 

 between G and H occupied the field of view, the retinal cir- 

 culation showed so plainly as to inconvenience me. On slightly 

 altering the focus, so as to get rid of the lines, I could see not 

 only the corpuscles, but the walls of the capillaries themselves 

 with great distinctness, especially when the eye began to be 

 fatigued. With a sufficiently narrow slit the corpuscles were 

 visible against the other parts of the spectrum, even the red, but 

 they appeared as mere specks, and the outlines of the vessels 

 could no longer be discerned. Judging from this that violet 

 light was more important than a narrow slit, I tried last year the 

 effect of various coloured media. Of these the most successful 

 was ammonio- sulphate of copper. A six-ounce flask, filled with 

 a solution of this substance, lield close to the eye about a yard 

 from an arc-lamp, enabled me to see the retinal circulation and 

 some of the vessels, but not nearly so distinctly as with the 

 violet light of the pure spectrum. George J. Burch. 



Oxford. 



JUPITER AND HIS PERIOD OF ROTATION. 



A QUESTION which has recently been attracting con- 

 -^^ siderable interest with regard to the greatest of our 

 planets, Jupiter, relates to the determination of the time 

 of rotation deduced from observations of markings on the 

 visible surface of this planet's disc. The difficulty to be 

 contended with here is that the appearance of the mark- 

 ings on the disc is always changing, and further great 

 changes, even in short intervals of time, are noticeable. 

 The chief characteristic features of the disc are the dark 

 belts situated on each side of the equator, resembling to 

 some extent the two belts of trade winds on the earth, 

 which lie on either side of the belt of equatorial calms 

 and rains. More minute scrutiny reveals to us other belts 

 which are sometimes seen in considerable numbers, while 

 their individual structure is by no means simple. These 

 details are for the most part only temporary, and so quickly 

 do changes take place, that the surface of the planet 

 seldom, if ever, appears the same two nights together. 

 Often spots are observed among these belts, which are 



NO. I 38 I, VOL. 53] 



generally of a more distinct nature than the usual mark- 

 ings ; these are for the most part dark, but in some cases 

 they are bright, round, and small, resembling the satel- 

 lites as they travel across the primary's disc. 



The extreme lack of constancy in the positions and 

 forms of all the surface-markings has long ago taught us 

 to cast aside the idea that we are looking at the surface of 

 a rigid body. What we really see is the dense vapour 

 and cloudlike formations between us and the real surface, 

 encircling the whole planet from pole to pole, and always 

 in a state of turmoil. This and other reasons have given 

 us sufficient evidence to form some idea of the tempera- 

 ture of the planet itself, and it is now thought that the 

 internal temperature is considerable, and probably 

 sufficient to render the planet capable of shining to a 

 small extent by its own light. 



In consequence of the proper motion of several spots 

 observed on Jupiter's surface and the primitive state of 

 his atmosphere, the planet is said to resemble the sun 

 to some extent. On account of these many points of 

 similarity, Zollner was led to believe that perhaps a 

 similar law of rotation might here be in vogue which 

 had been proved to hold good in the case of the sun by 

 observations of spots on his surface. Lohse also, some 

 time afterwards, after a minute discussion of all the 

 available evidence, came to a similar conclusion, namely, 

 that Jupiter in the region of his equator rotated quicker 

 than in regions some distance from it. 



Now this question is by no means so easy to settle as 

 may seem at first sight. In the first place, in the cases 

 of both the sun and this planet, the spots that are 

 generally visible are confined to two belts north and 

 south of the equator, while the equator and the poles are, 

 for the most part, devoid of all such markings. 



In the case of the sun, however, one need not necessarily 

 be restricted to spot observation to determine the time 

 of rotation near the poles, for we possess a very simple 

 means of solving this problem by the application of 

 Doppler's principle. The sun's limb on the east and 

 west sides will be rotating towards and away from the 

 observer, and an examination with the spectroscope of 

 these regions will show us, by measuring the displace- 

 ment of the lines in the spectrum from their normal 

 positions, the velocities m the two directions. Curiously 

 enough, both Crew and Duner made investigations on 

 this principle, and the results obtained were by no means 

 the same, but, on the contrary, apparently antagonistic. 

 The former, who observed the photosphere, found that 

 for all latitudes the time of rotation was the same, while 

 the latter, who examined the spots spectroscopicallyv 

 obtained a decrease in the velocity as their distance from 

 the equator increased. To explain these apparently con- 

 tradictory results, Brester suggested that it seemed 

 probable that Duner observed the spectral lines of gases 

 situated inside the photosphere, while Crew's observa- 

 tions were restricted in all probability to gases in those 

 atmospheric layers which lie above the photosphere. 



As yet no such observations near the poles of Jupiter 

 have been attempted, although, as will be seen further on, 

 this application of Doppler's principle has been employed 

 for the region near the equator. Fortunately, however^ 

 in the year 1892 (October 10) a short dusky streak,, 

 oblong in shape, was observed by Stanley Williams near 

 the north limb, extending nearly to north latitude 85°. 

 Other similar streaks have been subsequently noticed, 

 and frequent determinations of the times of mid-transit 

 have been made. By good fortune the observers at the 

 Lick Observatory had secured at the same time some 

 good photographs of the planet, the surface-markings of 

 which, on the negative, were sufficiently distinct for 

 measurement. 



A comparison of the times of rotation deduced from 

 both the visual and photographic records seemed to give 

 very satisfactory results, the mean periods differing only 



