16 Mr. Herschei/s and Mr. South's observations of the apparent 
alternately, as often as we please. The small companion of 
23 (h) Ursse Majoris, is a remarkable instance, and others 
will be found in the note.* The lateral portions of the re- 
tina, less fatigued by strong lights, and less exhausted by 
perpetual attention, are probably more sensible to faint im- 
pressions than the central ones, which may serve to account 
for this phenomenon. 
The measures were, for the most part, taken by both ob- 
servers in each other's presence, the one acting as assistant, 
and writing down what the other announced. Frequently, 
however, this disposition, dictated by convenience, was 
changed, and the observations made by one were read off, 
as well as written down, by the other, and the results not 
communicated till the measures were finished. This mode 
of checking each other's measures, the severest which can 
well be resorted to when two persons observe together, was 
however only adopted, when, from a discrepancy in the first 
measures, some suspicion of a bias in the eye, or judgment 
of one or other, arose, in cases of peculiar interest, or in 
the earlier part of the work, before practice had confirmed 
our confidence. When the two instruments were used at once 
however, which during the last year's observations was al- 
most perpetually the case, the observers were necessarily 
separated from each other, and their results only communi- 
cated on the following morning, at the time of taking and 
applying the index errors. 
In a very few instances, the assistance of a third person has 
been called in, to give a turn to our opinion in a doubtful case. 
Mr. Richardson, of the Royal Observatory, has generally 
* f Persei ; 7 Tauri ; 43 Persei; » Leporis (R. A. 5 h . 4 m .) ; 63 (p.) Geminorum. 
