Apeil 27, 1900.] 



SCIENCE. 



667 



of the paper was ' On certain Discontinuous and 

 Indeterminate Functions. ' 



E. D. Pebston. 



Secretary. 



DISCUSSION AND CORRESPONDENCE. 



BEQUEST FOR CO-OPBBATION IN WORK ON THE 

 COMING SOLAR ECLIPSE, ETC. 



There is a very singular plienomenon ob- 

 served during tiie moments immediately preced- 

 ing and following total solar eclipses, that has, 

 up to the present time never been satisfactorily 

 explained so far as I know. Just before totality, 

 usually about a minute before, alternate bright 

 and dark bands are observed sweeping across 

 the country. These have been called shadow 

 bands by some observers, and diffraction bands 

 by others. They can be observed to the best 

 advantage by laying a large piece of white cloth 

 on the ground. 



In some eclipse reports they are styled ' Dif- 

 fraction bands bordering the moon's shadow.' 

 Fringes bordering a shadow should, however, 

 move with the speed of the shadow. Observa- 

 tions show that the dark bands move quite 

 slowly, from ten to twenty feet per second, 

 while the shadow of the moon rushes across 

 the country at cannon ball speed. Moreover, 

 they move in one direction before the eclipse, 

 and in the opposite direction after. The only 

 half way plausible explanation that I have ever 

 heard offered for the shadow bands is that 

 they may be due to striae in the atmosphere. 

 This would bring thena under the head of the 

 scintillation phenomena treated of somewhat 

 extensively in advanced works on optics, but I 

 am unable to see how any such regular and 

 symmetrical distribution of light and shade can 

 result in this way. That the distance between 

 the bands varies on different occasions lends 

 some plausibility to this explanation, but it is 

 not impossible that the width of the bands is a 

 function of the location of the point of observa- 

 tion, that is to say of its distance from the 

 center of the eclipse track. This can only be 

 determined by numerous and extensive observa- 

 tions covering a wide tract of country, and it is 

 to secure as many data as possible on this sub- 

 ject that I desire to secure the co-operation of 

 all who are interested in the subject. Observa- 



tions just outside of and just within the track 

 of totality will be of especial interest. The 

 observations can be made without any appara- 

 tus, and as the bauds are not visible during 

 totality, their observation will not inconvenience 

 any who are more interested in the spectacular 

 than in the scientific side. At the end of this 

 article I shall outline as clearly as possible just 

 how the observations should be made, and 

 what data recorded. It has occurred to me 

 that the stroboscopic disc may be of use in de- 

 termining the cause of the bands. If a source 

 of light produces in any way, moving bands of 

 light and shade, it is obvious that if the eye be 

 directed towards the source, it will receive 

 more light from the source while a bright ba.nd 

 sweeps across it, than during the transit of a 

 dark band. If the alterations are not too 

 rapid a fluctuation in the brilliancy of the 

 source should be observed. 



As a matter of fact, citing a special case, the 

 bands are about three inches wide, and move 

 with a velocity of about ten feet per second. 

 This means that forty bands cross the eye every 

 second, too many to cause any flickering effect. 

 By means of a stroboscopic disc, which is merely 

 a circle of cardboard with equidistant radial 

 slits arranged to be rotated at varying speeds, 

 it is possible to keep the eye in a dark or light 

 band as long as we choose. 



Suppose we are looking at the source of light 

 through the slits of the revolving disc, and sup- 

 pose that the speed of rotation is such that the 

 slits cross the eye at the same rate that the dark 

 and light bands do. This is practically keeping 

 the eye continually in a dark or light band. If 

 the rotation is a little faster or a little slower, 

 the slits will alternately get into, and out of 

 step with the bands, and the eye will be in a 

 bright band one moment, and in a dark one the 

 next. In this way we may make the speed of 

 the fluctuations as slow as we please, and if we 

 look at the sun's crescent through such a device 

 we may possibly detect a flickering in what- 

 ever part of the source of light is operative in 

 producing the bands. The disc should be about 

 a foot in diameter with about eight slits in it, 

 distributed uniformly. I should advise that 

 three or four concentric rings of slits of different 

 width be made, the eye being moved from one 



