712 
north meridian mark, a small electric motor fur- 
nishing the motive power. The speed can be varied 
to represent that of any star from 0° to 89° of 
declination. The apparent magnitude of the star 
may be varied by interposing screens in the line of 
sight, or by changing the resistance in the electric- 
light circuit. 
This star will be observed by the use of the 
transit cirele, just as an actual star would be 
observed, the movement of the carriage carrying 
the star in the meantime causing a similar auto- 
matic record to be registered on the chronograph. 
A complete observation includes the observation 
of the star during its east and west movement, 
using a reversing prism at the eyepiece to keep 
the apparent direction the same. By comparing 
the observer’s record of such an observation with 
the automatic record, his absolute personal equa- 
tion can be determined, and by suitable series of 
observations, the personal equations of various 
observers dependent on velocity and direction of 
motion, or magnitudes of stars, and for such ob- 
jects as the limbs of the sun, moon, etc., may be 
determined. 
At present nearly all star catalogues are more 
or less affected by such errors. Even if a self- 
registering right ascension micrometer and a re- 
versing prism are used, by which many errors are 
greatly reduced, it is still desirable that observers 
should determine them, and if necessary apply 
corrections for them. 
Measures of the Satellite of Neptune, and of 
Oberon and Titania, Satellites of Uranus, made 
at the Naval Observatory, 1908-10: ASAPH 
HALL. 
After the publication, in 1875 and 1885, of the 
measures made at the Naval Observatory of the 
satellite of Neptune, Mr. Marth pointed out the 
curious motions of N and JZ, which determine the 
position of the satellite’s orbit plane with refer- 
ence to the equator. It has been explained that 
these motions might be produced by a flattening 
of the planet which causes the pole of the plane 
of the satellite’s orbit to describe uniformly a 
small circle about the pole of the planet. 
Therefore, for the purpose of following these 
motions, the satellite of Neptune has been meas- 
ured at the Naval Observatory during many oppo- 
Sitions. 
For the two oppositions 1908-09 and 1909-10, 
the following corrections have been obtained to 
the data of the Connaissance des Temps, which are 
the elements of H. Struve published in 1894: 
SCIENCE 
[N.S. Vou. XXXV. No. 905 
1908-09 
du==+1°.223+0°.190 
dN=-+1.341+0.383 adN=+1.097+0.313 
dI=-+ 0.4010.376 dI=—0.277+0.294 
G= lent Q=} 37°.34 
e= 0.002,71 e= 0.009,60 
da=-+ 0" .34407.056 da=-+ 0” .0580”.045 
1909-10 
du=-+ 0°.521+0°.162 
The corrections to u, N and I are believed to be 
teal. Evidently there is a considerable change of 
personal equation in the distance pointings, as has 
been shown already in the measures of several 
observers. 
The measures of Neptune’s satellite made some 
years ago at the Yerkes Observatory by Professor 
Barnard with the 40-inch refractor, give for the 
semi-major axis of the orbit at the mean distance 
of the planet from the sun, 167.22, instead of 
16”.27, which is usually accepted. 
On account of the large aperture of the tele- 
scope employed, this determination appears to be 
the most accurate of the visual measures, and least 
liable to systematic errors. 
Various experiments have been made at the 
Naval Observatory with reference to the elimina- 
tion of systematic errors, including the use of re- 
versing prisms. However, with the prisms em- 
ployed, so much light is lost that they can be used 
only on very good nights. 
For Oberon and Titania, satellites of Uranus, 
the following corrections have been obtained to 
the elements of the Connaissance des Temps from 
observations made at the oppositions of 1908, 
1909, 1910: 
Oberon 
du==-+ 0°.767+0°.207 
adN=——0.077=0.360 
dI—=—0.611+0.347 
Q@= 218°.41 
e= 0.0100 
da=-+ 0”.167+07.119 
Titania 
du=-+1°.340+0°.262 
adN=—0.634+0.474 
adI=—0.730+0.412 
Q= 216°.45 
e= 0.0934 
da=-+0".13907.090 
From measures secured in 1911 by Mr. Eppes 
and Mr. Burton, Mr. Eppes has found the follow- 
ing corrections to the data of the Connaissance 
des Temps: 
Oberon 
du=-+ 0°.735+0°.179 
adN=—0.263+0.261 
dI==-+0.221+0.235 
Q= 60°.06 
e= 0.00214 
da=-+0".0720”.074 
Titania 
du=+1°.778+0°.159 
adN=—0.253+0.295 
dI=-+0.033+0.271 
Q= 285°.02 
e= 0.00180 
da=-++0".2080”.075 
