456 PHYSICAL SCIENCES. 



focus of the eye-piece, so that the middle wire is perpendicular and at right 

 angles to the axis of the telescope. It consequently represents a portion of 

 the celestial meridian ; and when a star is seen to cross that wire it then 

 crosses the celestial meridian of the place of observation. A clock beating 

 seconds being close at hand, the duty of an observer is to note the exact 

 second and part of a second at which a star crosses each wire successively 

 in consequence of the rotation of the earth. Then the mean of all these 

 observations will give the time at which the star crosses the celestial 

 meridian of the place of observation to the tenth of a second, provided the 

 observations are accurate. Now it happens that the simultaneous impres- 

 sion on the eye and ear is estimated differently by different observers, so 

 that one person will note the transit of a star, for example, as happening 

 the fraction of a second sooner or later than another person ; and as that is 

 the case in every observation he makes, it is called his personal equation, 

 that is to say, it is a correction that must be applied to all the observations 

 of the individual, and a curious instance of individuality it is. For instance, 

 M. Otto Struve notes every observation 0"-11 too soon, M. Peters 0"'13 

 too late ; M. Struve noted every observation one second later than M. Bessel, 

 and M. Argelander estimated the transit of a star 1"'2 later than M. Bessel. 

 All these gentlemen were or are first-rate observers ; and when the personal 

 equation is known it is easy to correct the observations. However, to 

 avoid that inconvenience Mr. Bond has introduced a method in the Obser- 

 vatory at Cambridge in the United States in which touch is combined 

 with sight instead of hearing, which is now used also at Greenwich. 

 The observer at the moment of the observation presses his fingers on a 

 machine which by means of a galvanic battery conveys the impression to 

 where time is measured and marked, so that the observation is at once 

 recorded and the personal equation avoided. 



NOTE 149, p. 84. Let N be the pole, fig. 11, e E the ecliptic, and Q q 

 the equator. Then, N n m S being a meridian, and at right angles to the 

 equator, the arc op m is less than the arc cyo n. 



NOTE 150, p. 85. Heliacal rising of Sirius. When the star appears 

 in the morning, in the horizon, a little before the rising of the sun. 



NOTE 151, p. 87. Let P cyo A ./v, fig. 35, be the apparent orbit or 

 path of the sun, the earth being in E. Its major axis, A P, is at present 

 situate as in the figure, where the solar perigree P is between the solstice 

 of winter and the equinox of spring. So that the time of the sun's 

 passage through the arc cyo A ./v is greater than the time he takes to go 

 through the arc _Q. P qp. The major axis A P coincided with .n. op, the 

 line of the equinoxes, 4000 years before the Christian era ; at that time P 

 was in the point cyo. In 6468 of the Christian era the perigee P will 

 coincide with .A.. In 1234 A.D. the major axis was perpendicular to op .A., 

 and then P was in the winter solstice. 



NOTE 152, p. 88. At the solstices, $c. Since the declination of a 

 celestial object is its angular distance from the equinoctial, the declination 

 of the sun at the solstice is equal to the arc Q e, fig. 1 1 , which measures 

 the obliquity of the ecliptic, or angular distance of the plane op e .A. from 

 the plane cyo Q -A_. 



NOTE 153, p. 88. Zenith distance is the angular distance of a celestial 

 object from the point immediately over the head of an observer. 



