IOO CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



would be no empirical corrections in the Nautical Almanac 

 to the semidiameters of the Sun and Moon ; and the diame- 

 ters of the planets measured by different observers, and 

 the right ascensions and delineations of the stars would be 

 consistent within the instrumental and personal equations. 



Other Important Results Affected. 



There is another and very important class of astronomical 

 results affected by these factitious borders of the Sun and 

 planets and the unsteadiness of the stars. 



Dr. Newcomb says that the observed right ascensions of 

 the mean of the Sun's two limbs, relative to the fixed stars, 

 are affected by personal errors for which no means of elim- 

 ination have been tried. He suggests personal error and 

 possibly the effect of the Sun on the telescope. To these 

 causes should be added the larger effects of the disturbed 

 and factitious limbs; and in cases of steadiness, to diffrac- 

 tion of the limb at the spider line. He elsewhere notes 

 "very large errors, both accidental and systematic, to which 

 observations of the Sun are liable;" and again he regards 

 "the constant error in declination of the Sun as something 

 peculiar to the observer and the instrument." In another 

 place he says: "There is a remarkable systematic differ- 

 ence in the observed A. R. of Mercury, according as the 

 planet is east or west of the Sun, and therefore according 

 to the illuminated side. The sign of the result shows that 

 the reduction to the center of the planet was apparently too 

 small, and then it is of interest to learn according to what 

 law this error changed as the planet moved around its rela- 

 tive orbit." 



Furthermore, the hurtful effect of observing upon the 

 outer limb of the factitious limb of the disturbed Moon can 

 be shown in the determination of the parallactic inequality 

 of the Moon from meridian observations. Dr. Newcomb 

 says the method is peculiarly liable to systematic error, 

 owing to the fact that observations have to be made on one 

 limb of the Moon when the inequality is positive, and on 

 the other limb when it is negative. Hence*if we determine 



