1875.| 
(387) 
CORRESPONDENCE. 
THE POLE STAR AND THE POINTERS. 
By Capt. ALLAN CunNiINGHAM, R.E., Hon. Fell. of King’s Coll., Lond. 
Srr,—Attention has been drawn (in 
No. xliii. of the ‘“‘ Quarterly Journal 
of Science) to an interesting pheno- 
- menon conneé¢ted with the well-known 
rule for finding the Pole Star by 
means of the “ Pointers” (a and ( 
Ursz Majoris). It is there asserted 
that it is matter of popular observa- 
tion that the ‘‘ Pointers ” do not at all 
times appear to * point” equally well 
to the Pole Star, and an attempt is 
there made to prove that popular 
observation is herein correct in fact, 
that is to say that the ‘“‘ Pointers” 
do not actually at all times ‘‘ point” 
equally well to the Pole Star. 
The present writer, however, con- 
siders that the true explanation of 
the phenomenon has been quite 
missed in the Article referred to, and 
that the Article itself, indeed, con- 
tains its own refutation. 
The true explanation to the present 
writer’s mind is that— 
1. The Pointers do not actually 
at all times (of the same 
night) ‘* point ” equally well 
to the Polar Star by an 
amount depending solely on 
aérial effects of refraction. 
2. The Pointers do not appear— 
to the unaided human eye— 
to “ point ” equally well at 
all times (of the same night) 
to the Pole Star by an 
amount depending solely on 
an error of judgment (of the 
observer) in tracing the *‘ di- 
recting line” in the heavens, 
and which apparent change 
of the obliquity of the 
** directing line’ must there- 
fore be considered an OPTICAL 
ILLUSION. 
3. There is a minute annual 
change of the ‘directing 
line ”—due to the earth’s 
orbital motion—appreciable 
only with difficulty by the 
best instruments. 
4. There is a slow secular 
change in the relative posi- 
tions of the three stars—due 
to their proper motions— 
in consequence of which the 
“directing line” of the 
Pointers will in the course 
of ages cease to ‘ point’’ 
towards or near the present 
Pole Star, but this change 
will be very slow. 
The first two effe@s of actual and 
apparent change (during the same 
night) of the obliquity of the ‘* dire&- 
ing line” of the Pointers are the only 
ones of present interest. 
It may be premised that both 
effects will be zero at the North Pole, 
and extremely small in high northern 
latitudes; and will both increase 
with proximity to the Equator, the 
latter increasing much the more 
rapidly. 
The former effect — the actual 
change—would be so small in Great 
Britain as to be appreciable only with 
instruments, but would probably be- 
come appreciable to the unaided eye 
in low northern latitudes: the latter 
effect—the apparent change (merely 
an optical illusion)—will rapidly in- 
crease with distance from the North 
Pole, and is perceptible in Great 
Britain. 
As both effects increase with prox- 
imity to the Equator, the joint effe@ 
would of course be more noticeable . 
in low northern latitudes, especially 
to one leaving a considerably higher 
northern latitude; indeed the atten- 
tion of the writer of the previous 
Article seems to have been first 
attracted (see p. 286 of No. xliii. of 
the ‘* Quarterly Journal of Science ”’) 
to the phenomenon when ona voyage 
towards the Equator. 
That the principal part of the 
apparent change of obliquity of the 
** directive line”’ of the Pointers from 
the Pole Star is really an error of 
judgment—i.e. only an apparent 
change, or an optical illusion—may 
be with some trouble experimentally 
verified by any one who has a small 
