Febeuaby 11, 1910] 



SCIENCE 



227 



marked polarization that is a maximum in a 

 direction at right angles to the incident radia- 

 tion. It is true that we have in the turning 

 of the tail always away from the sun strong 

 evidence (since this is due, we believe, to light- 

 pressure) of the minute size of the luminous 

 particles; but, nevertheless, such evidence as 

 the phenomena of polarization can give on this 

 point is worth having. 



It would also be desirable to determine the 

 relative amount of reflected to intrinsic light, 

 though the method of accurately doing this is 

 not obvious. 



Polarization work can be done with any 

 refracting telescope of large light-gathering 

 power. A reflecting telescope could not be 

 used for this purpose because of the polariza- 

 tion effects that it itself would introduce. 



{d) Light-fluctuation. — It is well known 

 that the light of comets often varies irregu- 

 larly and without obvious cause. These varia- 

 tions should be studied in connection with the 

 formation of jets and envelopes, and especially 

 observed to see where the changes in brilliancy 

 have their origin and how rapidly they spread 

 to other parts. 



The position and size of sun spots, and other 

 solar phenomena, should also be observed and 

 studied in connection with the light changes. 

 Evidently the luminescence of comets is, in 

 some way, largely dependent upon the sun, 

 and it has been claimed that it is greatest 

 during periods of sun-spot maxima. If so, 

 then it may change with the size and orienta- 

 tion of the spots. At any rate, this is a phe- 

 nomenon that can easily and, perhaps, profit- 

 ably be studied with the aid of even a very 

 modest equipment. 



All the above phenomena can be observed at 

 any time the comet is brightly visible, but 

 there are a number of other phenomena which 

 possibly may appear or be modified during the 

 passage of the earth through its tail, if, for- 

 tunately, such an occurrence should happen, 

 and which, therefore, ought to be carefully 

 "> watched at that time. These form the second 

 or terrestrial group, above mentioned, some of 

 which are: 



1. Electrical Potential. — In reality it is the 



difference between the electrical potentials of 

 two points a given vertical distance apart in 

 the atmosphere that is here referred to. This 

 would be modified by the bringing of an elec- 

 trical charge from some extraneous source to 

 the atmosphere, and, conceivably, might there- 

 fore help to give some idea of the electrical 

 condition of that part of a comet's tail through 

 which we happened to pass. But, as the elec- 

 trical state of the atmosphere changes so 

 greatly from place to place and from day to 

 day, it does not seem that observations of this 

 nature can afford much definite information. 



2. Atmospheric Conductivity. — This comes, 

 essentially, to the same thing as the ionization 

 of the atmosphere, and would be modified by 

 the entrance into the air of charged particles 

 or other ionizing agents. 



Like the electric potential of the air this too 

 is subject, ordinarily, to such changes that, 

 seemingly, no trustworthy inference in regard 

 to the electrical condition of a comet's tail, 

 should we pass through one, could be drawn 

 from such observations. 



However, if any one, not entirely familiar 

 with them, wishes to take up either or both of 

 these lines of work he will find Gockel, " Die 

 Luftelektrizitat," a good guide. 



3. Damping of Electrical Waves. — It is well 

 knovm that the distance a wireless message 

 can be received changes irregularly, owing, 

 presumably, to the intensity and distribution 

 of the ionization of the atmosphere. The ease 

 or difficulty of transmitting wireless messages, 

 especially over the ocean, say from San Fran- 

 cisco to Honolulu, might, therefore, give some 

 hint about the electrical state and the ionizing 

 action of the material of a comet's tail through 

 which the earth at that time might chance to 

 be passing. Probably the hint would not be a 

 very distinct one, but observations of this 

 phenomenon seem to the author much more 

 promising of results than do those of either 

 the potential or the conductivity of the 

 atmosphere. 



4. Earth Currents. — A marked change in 

 the electrical condition of the atmosphere is 

 likely to lead to earth currents of greater or 

 less magnitude. It might therefore be well to 



