388 
portable’ equatoriably mounted tele- 
scope (a ‘‘ comet-seeker’’ would do 
very well)—it would be better if fitted 
with clockwork motion—or a small 
transit theodolite. The experiment 
would probably be more convincing 
to many than any amount of purely 
geometrical reasoning as to what 
ought to be. It will be described as 
performed with a small equatorial 
fitted with a set of hairs in its dia- 
phragm, and means of illuminating 
the hairs. 
Experiment.—The instrument must 
be set up so that the centre of the 
middle hair shall—by moving the 
telescope round the declination 
axis only—traverse both Pointers: 
the proper position will differ only 
slightly from the usual position of an 
equatorial, and may be found—by 
trial—by setting up the instrument 
as usual, z.e. with the polar axis 
pointing to the North Pole, and then 
correcting the position by shifting the 
polar axis, partly in azimuth, partly 
in altitude. The correct position, 
viz. that in which the centre of the 
middle hair traverses both Pointers 
on rapidly moving the telescope round 
its declination axis, will be secured 
after a few trials. When this posi- 
tion has been secured, the reading of 
the hour-circle should be noted: 
after which, turn the telescope round 
its declination axis, as near the Pole 
Star as possible—when, if the tele- 
scope be of low power, the Pole Star 
will be in the field of view: the 
centre of the middle hair must now 
be made to cover the Pole Star by 
use of the motions round both polar 
and declination axes ; after which the 
hour-circle must be read again. One 
observation is now complete: the 
difference between the two readings 
on the hour-circle will be the 
deviation (p m in Fig. 1) of the 
“ direGtive line” of the Pointers from 
the Pole Star—affected, however, 
to some extent by the rotation of 
the earth during the interval of 
observation. 
It is very desirable that the whole 
observation—from the moment the 
instrument is in proper position— 
should be rapidly performed, so as to 
reduce the last effeét as much as 
possible: the apparent motion of the 
Pole Star is so slow, that, if the 
operation be rapidly performed, the 
Correspondence. 
[July, 
earth’s rotation will sensibly 
affect the observation. 
The above process should be re- 
peated at other times of night: the 
position of the instrument will require 
slight shifting for every observation 
to secure the necessary condition 
that the centre of the middle hair 
should traverse both Pointers by the 
rapid motion of the telescope round 
the declination axis. 
It is obvious that a transit theodo- 
lite might (with some trouble) be 
used in the same manner by fitting it 
with a strong universal joint and 
clamp, so that it might be possible to 
tilt its horizontal plates into the same 
position as the hour circle of the 
equatorial. Care must be taken that 
the joint and clamp are strong 
enough and stiff enough to hold the 
whole weight of the instrument 
steady throughout the observation. 
It will be found that the values of 
the ‘deviation’ of the ‘‘ directive 
line’? or the Pointers obtained from 
different observations will—if the 
observations have been well made— 
not differ from each other by any 
such large amount as would be appre- 
ciable by the unaided eye. 
The result of this experiment is a 
proof positive that the principal part 
of the apparent change of obliquity of 
the ‘directive line” is only an 
apparent change, i.e., an optical illu- 
sion. 
The small difference in the values 
of the ‘‘ deviation ” as obtained from 
different observations, are due to the 
following causes:—r. An_ actual 
change due to aérial effects of refrac- 
tion. 2. Errors of observation due 
partly to errors of the observer, and 
partly to the imperfection of the 
experiment in introducing improperly 
effects of the earth’s rotation. 
It will now be shown that the 
above result might have been ex- 
pected @ priori, i.e., that—barring 
effects of refraction —there is no 
change in the obliquity of the 
‘* directive line” of the Pointers from 
the Pole Star at different times or 
places. 
Consider first the meaning of the 
word “ point’ as here used of stars. 
The popular explanation would pro- 
bably be— 
Populay Explanation. — ‘‘ Two 
stars are said to ‘point’ at 
not 
