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 From- 

 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 DE GLASENAPP 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. DOWNING in "The Observatory", Vol. XII. 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 .Fraw-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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