30 



KNOWLEDGE. 



[December 2, 1889. 



it must not be supposed that the former are merely 

 youi]ji;er forms of the latter. A Hea has not throughout its 

 life the form with which we are familiar, nor does it in 

 that form grow at all. The little fleas are simply the 

 males, which are considerably smaller than the females, 

 in accordance with a rule very frequently illustrated 

 amongst insects. 'Lire males also differ in shape, and 

 have the hinder end of the body somewhat turned 

 upwards. In its life history a flea diflers totally from a 

 bug ; the former is an insect with a complete metamor- 

 phosis, and therefore altogether differently shaped in its 

 larval condition, while the latter is almost identical in 

 shape during the whole of its life, and exhibits similar 

 habits throughout ; hence the tiny bugs are really young 

 ones, though this is not the case with fleas. 

 / To be continued, i 



ON THE SCINTILLATION OF STARS. 



By a. C. Eanyari). 



IF a large star near to the horizon is watched by an 

 ordinary long-sighted person,* it will be noticed that 

 its light is not steady, but that three classes of 

 changes are continually taking place. The bright- 

 ness of the star increases and decreases. Its light 

 ■changes in colour, and the central bright point appears to 

 dance or jump about. All these changes put together 

 make' up the phenomenon known as scintillation or 

 twmkling, v/hich is so evident, even at considerable alti- 

 tudes, that most people are able to recognise planets by 

 the steadiness of their light which does not scintillate or 

 twinkle. 



In order to see the changing colours to the best advan- 

 tage, let anyone direct a small telescope to a star not very 

 far from the horizon, and, having focused it sharply, let 

 him tap the eye-piece or body of the telescope so as to 

 make it vibrate. To the observer the star will appear to 

 move, and not the telescope. It will appear to be ih'awn 

 out into a line of light as the eye follows the motion of the 

 edge of the field of the telescope, and the star is projected 

 upon different parts of the retina. If the telescope is a 

 small one, and vibrates rapidly so that the period of its 

 swing does not occupy more than about a third of a second, 

 the star will appear to describe a closed orbit, generally 

 either an ellipse or a circle, along which tlie different 

 colours repeat themselves very brilliantly. M. Montigny 

 of Bruxelles has utilised this method for comparing the 

 amount of scintillation on different nights, and for the 

 same stars at different altitudes. His mstrument, which 

 he calls a scintillometer, enables him by whirling round a 

 small disc in front of the eye-piece to make the image of 

 a star appear to describe a circle in the field of the tele- 

 scope, upon the circumference of which he counts the 

 number of times that the colours are repeated ; and the 

 rate of motion being known, he calculates the number of 

 changes of colour per second. 



* I am careful to restrict the description to what is seen by long- 

 sighted people, for short-sighted people see the stars as discs or 

 patches of li^ht on the sky. They see a bright disc fi>r a laige star, 

 and a duller disc of about the same diameter for a small star. Many 

 people go through life without kncwing that the stars are seen by 

 their neighbours as points of light, .and to such people the phenomena 

 of scintillation are mucli less perceptible. If a long-siglited person 

 wishes to realise how a short-sighted person sees the heavens, let 

 him take an opera-glass and look at the stars with it when out of 

 focus, and a short-sighted person may realise how a long-sighted person 

 sees the stars by focusing tlie opera-glass until the star images all 

 appear as bright points. He will then be able to see the phenomena 

 of scintillation as described above. 



Authorities differ as to the exact cause of .scintillation. 

 Let us, therefore, examine closely the facts that may be 

 observed, and study the conditions under which the light 

 of a star reaches us. If the spectrum of a star rising in 

 the east be examined, darkish bands are seen rapidly 

 traversing the spectrum from the blue to the red ; and, 

 on the other hand, in the spectrum of a star setting in tlie 

 west the dark bands traverse the spectrum from the red 

 to the blue end. When a star is on the meridian the 

 bands sometimes traverse the spectrum in one direction, 

 and sometimes in the other, and sometimes they swing 

 alternately to and fro. We evidently have in the regular 

 motion of the bands in the light of a rising or setting star 

 a phenomenon coimected with the earth's motion on its 

 axis ; while in the irregular motion of the bands in the 

 spectrum of a star on the meridian we seem to have a 

 phenomenon connected with tlie direction in which the 

 winds are blowmg. If there are masses which cause 

 unequal refraction in the air, they would cause the light of 

 the star to be concentrated in some places and turned 

 away from others. If we consider only one of the rays of 

 the spectrum, say a blue ray, and the light of the star 

 were strong enough, it would illuminate a surface much 

 as the shallow bottom of a rippling pool is lit up by the 

 sun, causing light and dark mottlings, which appear to 

 move along with the ripples on the surface. Each ray 

 will be diff'erently refracted, and consequently the blue 

 and the red mottlings will overlap and be constantly 

 changing. If we trace backward from the eye the different 

 rays of the spectrum through the air it will be seen that 

 the violet ray, which is most refracted, must have entered 

 the air at a greater height than the red ray, and we may 

 calculate the distance between the two rays at various 

 altitudes. The different behaviour of the two rays shows 

 that many of the refracting masses in the air are small 

 compared with the separation of the dift'erent coloured 

 rays, and that the refraction causing the dark bands in 

 the spectrum must take place at a considerable altitude. 

 This also is evident from other considerations. 



The various coloured rays from the star are carried 

 athwart the refracting masses as the star rises, and the 

 violet ray will be the first to be affected by any particular 

 refracting mass — it will then successively aff'ect the blue, 

 the green, the orange, and the red, and, as we have seen 

 with a rising star, a dark band passes down the spectrum 

 from the violet to the red end. We can calculate the rate 

 at which these rays sweep across a refracting mass at any 

 distance from the observer — as was explained in the 

 article on Mountain Observatories in the April number, a 

 line from a star to the eye of the observer sweeps onward 

 like the hand of a great clock, which makes one revolution 

 in twenty-four sidereal hours. At a distance of 25 

 miles from the observer the thwart motion of the rays 

 through the air (if no wind were blowing) would be about 

 9 feet 7 inches per second, or a little more than 6 

 miles an hour — a velocity wliich is small compared 

 with the average velocity of the wind in the upper air. 

 Thus M. Angot gives the average velocity of the wind at 

 the top of the Eiffel Tower — 994 feet above the ground — 

 as 1(3 miles per hour, while the average velocity during 

 the 101 days of observation, measured with a similar 

 instrument placed GO feet above the grotmd at the 

 Paris Meteorological Oiiice, was only 5 miles an hour. 

 At greater heights the wind velocity is probably still 

 greater, for example, we Imow that the dust from Krakatoa 

 was carried to Trinidad, half round the earth, in seven 

 days, showing an average westerly velocity of 80 miles 

 mOes per hour. The regular drift of the dark bands in 

 the spectrum of a rising or setting star shows that the 



