2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 76 



no use of observed solar phenoniena, and depending in all proba- 

 bility upon a false eclipse cycle. 



This paper is not intended to traverse the extensive ground covered 

 by Danjon ; the period covered is 1860-1922, the former date having 

 seemed natural, as about that time began the serious application of 

 the spectroscope to astronomy, and also the publication, in the British 

 Nautical Almanac, of more complete elements, convenient for making 

 projections of eclipses. 



It was soon obvious that spectroscopy has played but a limited 

 part in the study of lunar eclipses. And so of instrumental photometry ; 

 determinations of the brightness of the eclipsed moon, except by 

 rough comparison with stars, have been few. And so too of photog- 

 raphy; while innumerable photographs of the eclipsed moon have 

 been taken, apparently little use has been made of the plates. Almost 

 all observations have been made with plain telescopes of all sizes, 

 and with the naked eye ; the brightness of the moon has seemed 

 secondary to the moments of contact of the umbra-edge with the 

 moon's limb and with various lunar objects. Seeliger ^ having shown 

 that in all probability the apparent enlargement of the shadow 

 detected by such observations has no objective importance, it is 

 easy to agree with Crommelin that there is little use in piling up 

 more of them. 



Due to well-known optical principles, the telescope does not increase 

 the brightness of any extended area, but really diminishes it. How- 

 ever, it is a fact of observation that during a lunar ecHpse, objects 

 which may be seen in the shadow with large telescopes are invisible 

 with smaller. It is like the effect of night glasses ; or like the fact 

 that newspaper headlines in twilight can be read when the text below 

 is a blurred mass. This suggests a criterion for the brightness of 

 eclipses, usable when the observations are simply telescopic or with 

 the naked eye. 



On account of the mixed quality of the data a simple three-step 

 scale of brightness has been adopted. 



Grade 2. — When the naked eye sees the " spots " on the eclipsed 

 moon, or the seas and other detail can be seen with hand instruments — 

 opera-glasses, field-glasses, spy-glasses. 



Grade i. — When instruments of aperture of 2 inches up to 6 inches 

 are necessary to show detail on the eclipsed surface. This includes 

 ordinary stand-telescopes, porch telescopes, etc., and some fixed 

 observatory telescopes. 



* H. Seeliger, Abh. Akad. Wiss. zu Miincheii, II KL, 19 II, pp. 383-448, 1896- 

 97 ; Die scheinbare Vergrosserung des Erdschattens bei Mondfinsternissen. 



