222 DIRECTIONS FOR OBSERVING 



But to save observers from fruitless trials, and to place at their 

 service tlie experience which I have acquired from the numerous ob- 

 servations made at Merges, as well as to render the indications of one 

 observer comparable with those of another, I tliink it proper to 

 recommend the following instructions: 



1. Planner of observing. — I have tried different scintillometres. all 

 those indicated by Arago, and even a new one proposed by myself; 

 but I have come definitively to the conclusion that for oi3serving, 

 none of them are equal to the naked eye. 



It is easy, when one is a little accustomed to it, to judge with suffi- 

 cient exactness whether one star scintillates more than another, and 

 the scintillation may be indicated by a number, as in meteorology we 

 mark by a number the state of the sky or the force of the wind. For 

 my own use I have designated by an absence of scintillation, and 

 by 10 one of those strong scintillations which are seldom witnessed 

 except when the star is near the horizon, when it appears to scin- 

 tillate, change color, or even disappear. With a little practice, we 

 succeed even in distinguishing degrees between the scintillations 

 and 1 and 1 and 2, <fec.; we can then appreciate the scintillation with 

 more exactness, and designate it, for instance, by 0.4, 1.6, &c. It 

 is scarcely possible, however, to carry these subdivisions of gradation 

 beyond the scintillations 4 or 5. 



This is my own scale ; another observer may frame such an one as 

 he thinks suitable. The figures may vary, but the relations will re- 

 main essentially the same; thus my brother Marc, who has been 

 engaged nearly a year in observing the scintillation, employs a much 

 lower scale, yet we are generally in accord as to the relation of the 

 numbers; we are always so in regard to the question whether one 

 star scintillates more or less than another, and whether the scintilla- 

 tion during one evening is stronger or more feeble than during another 

 evening. Now, this is the important point, for since all the observa- 

 tions cannot be made by the same person, it will be often difficult to 

 know if the scintillation is exactly of the same intensity at Pulkova, 

 at Archangel, on the Peak of Teneriflfe, or at Morges. 



I shall show presently, however, how these inconveniences can 

 be obviated within certain limits.. But in all cases, by following the 

 mode of. observation just indicated, it will be possible to know how at 

 each of these stations the scintillation varies from one day to another, 

 and whether this variation appears to bear a relation to any meteoro- 

 logical perturbation. 



It is scarcely necessary to add that the height of the star which 

 scintillates must always be known, or at least be capable of determi- 

 nation. But in place of observing this height directly, it is more 

 simple to calculate it from the hour of the observation, and, to abridge 

 the calculation, a table may be prepared in advance, indicating, with 

 reference to the latitude of the station, the height of the stars ob- 

 served at different sidereal hours. My own table gives these heights 

 for Morges from half hour to half hour. This table answers the pur- 

 pose, for in the interval of a half hour one may interpolate Avith all 

 the necessary exactness, since it is useless to know the height of a star, 



