252 CAMPBELL— CROCKER ECLIPSE EXPEDITION. 



obtain improved data as to the limiting depths of the strata respon- 

 sible for the various solar absorption lines. 



Contact Times. 



The times of contact of sun and moon have been more exten- 

 sively observed than any other eclipse phenomena. It has long 

 seemed to me that the first and fourth contacts, that is, the instants 

 of time when the moon's image first touches the sim's, and when 

 the moon's image finally leaves the sun's, are not worthy of much 

 attention, and our expeditions have taken no interest in them. The 

 time of the first contact is bound to be uncertain. The observer 

 who is watching for it suddenly realizes that the moon has covered 

 a bit of the sun. How many seconds earlier the contact really oc- 

 curred he does not know. He is simply aware that it has occurred, 

 and several seconds of uncertainty are unavoidable. The fourth 

 contact can be observed with considerably more accuracy, but a first 

 contact of equal accuracy does not exist to balance it. The second 

 and third contacts, on the contrary, can be observed quite accurately. 

 Nevertheless, the brilliant points continuing to exist at contact 11. 

 after the general outline of the moon's edge has passed beyond the 

 sun's corresponding edge, owing to depressions in the lunar sur- 

 face — and similarly at contact HI. — introduce some uncertainty. It 

 seems to me that very valuable comparisons of the relative positions 

 of the sun and moon could be obtained by a series of large-scale 

 photographs, using cameras forty feet or more in length, made 

 near contacts II. and III., under conditions carefully devised for 

 reducing the brilliancy of the solar crescents and for the accurate 

 recording of the observation times. 



The Accurate Position of the jNIoon. 



It is very advantageous to eclipse observers that meridian obser- 

 vations of the moon's position be made in months immediately pre- 

 ceding the eclipse date, as a basis for predicting the time when to- 

 tality will begin at any station on the shadow path. The eclipse of 

 1918 came seventeen seconds of time earlier than the Nautical Al- 

 manacs had predicted three years in advance, three seconds later 



