Vol.. 8, 1922 
ASTRONOMY: G. E. HALE 
169 
ternate extinction of its red and violet edges. In this way magnetic 
fields of an intensity of 200 gausses can be detected. 
A systematic search for invisible spots was begun by Mr. Ellerman 
and the writer on November 19, 1921, when two were found, with nega- 
tive magnetic fields of 400 and 300 gausses, respectively, in the calcium 
flocculi following a positive single spot. No. 1920. One of these may not 
have belonged to this spot group, as it was about 4° south. Both were 
again detected by Mr. Ellerman on the following day, with slightly weaker 
fields. On November 21 only one could be found, and its position was 
now indicated by a faint marking, which on the following day became a 
visible spot, with a field-strength of 300 gausses. The other invisible 
spot to the south was not found, but another spot, preceding it and not 
previously detected magnetically, had also appeared. 
Several other invisible spots have since been observed. The two small 
spots constituting the following member of No. 1919, which disappeared 
to the eye on November 25, were still detected magnetically by Nicholson 
on November 26 and on November 27, with positive fields of 300 and 200 
gausses respectively. No. 1924, recorded from visual observations as a 
positive unipolar spot, was found by Ellerman on December 12 to be fol- 
lowed by a negative magnetic field of 200 gausses; both this and another 
invisible spot of negative polarity, observed by him on the following day, 
appeared as visible spots on December 14. On January 9, 1922 Ellerman 
observed a positive invisible spot (300 gausses) following the eastern nega- 
tive spot of a regular bipolar group, No. 1932. No spot was observed in 
this position either before or after January 9. A small positive spot which 
appeared in group No. 1938 on January 23, 1922, though not observed 
either visually or magnetically on the two following days, was detected 
b)y Ellerman as a positive invisible spot (400 gausses) on January 26. 
T'ollowing the unipolar spot No. 1942 an invisible negative spot (300 
gausses) was observed on February 6 by Ellerman and on February 26 
he found a positive invisible spot (500 gausses) following No. 1945 (nega- 
tive) and a negative one (300 gausses) following No. 1947 (positive). 
No visible spots were seen in either of these positions. On March 15 
Nicholson detected a negative invisible spot (300 gausses) following No. 
1953 (positive) in the region where the following spots had disappeared. 
In an examination of our earlier magnetic records we have found nine 
other cases in which a local magnetic field was observed where no spot 
was recorded. Most of these were probably invisible spots, but as very 
small visible spots at the points in question might have escaped notice, 
and as the magnetic observations were not followed up, they are not in- 
cluded here. 
The systematic observation of invisible spots, especially during the 
periods preceding and following the visible life of those that reach ma- 
