No. 6.] INTRODUCTION. CHRONOMETERS. XXV 
weak point in the numerical part of the foundations. It is also expressly 
stated by Damoiseau that some of his constants require further investigation. 
In order to deduce empirical corrections which can be used for the Fram- 
observations it was first necessary to reduce the continental observations to 
some common standard in regard to extraneous circumstances. It is well 
known that the treatment of observations of these eclipses is difficult. Some 
25 years ago Professor pe Guasenapp of St. Petersburg made an elaborate 
investigation principally with the intention of deducing the light-equation from 
a large series of observations of Sat. I. By the courtesy of the author I am 
in possession of the original memoir, but as I am quite unacquainted with 
the Russian language, my knowledge of its contents rests on a very clear 
abstract given by Mr. Downine in “The Observatory’, Vol. XI. It was 
necessary for the author’s purpose to take into consideration: the aperture of 
the telescope, the absorption of light by the atmosphere and its dependence on 
the altitude, the Planet’s distance from the Earth, the excentricity of Jupiter's 
orbit, the phase, the Satellite’s angular distance from the Planet at the time 
of reappearance or disappearance, and the effect of the penumbra. The final 
result is not encouraging for the treatment of such observations. After having 
deduced the light-equation and two other quantities from the observations, 
reduced to a common standard in regard to the circumstances named above, 
Mr. de Glasenapp had the happy idea to solve his equations afresh, using the 
observed times as they stand. The probable errors in this latter case are not 
much greater than in the first, which means that the discordances between 
the predicted and the observed times of disappearance and reappearance of 
Sat. I may, to a large extent, be considered as accidental. 
For the purpose of utilising the Hram-observations the case is so far 
different that there is no question about the absolute moment of the Satellite’s 
centre being on the limb of the shadow, and that the outer Satellites are of 
the same importance as Sat. I. As the telescope used on board was consi- 
derably smaller than those generally used in observatories for the same ob- 
servations, it was necessary to take account of the aperture; and it must also 
be admitted that the Planet's distance from the Earth may have a sensible 
effect on the magnitude of the “invisible segment’, i. e. the illuminated por- 
tion of the Satellite’s disc which is at the limit of visibility for a given tele- 
scope. As to the absorption of light at different altitudes, the writer was in 
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