MONTHLY MAGAZINE. 
Wie: 400. | 
SEPTEMBER 1, 1824. 
[Price 2s. 
For the Monthly Magazine. 
Tables shewing the Proportionate Re- 
fraction of the Light of several fixed 
Stars and Planets, and its Dependance 
on the Proportions of their coloured 
Rays. By T. Forster, M.D. F.L.S. 
M.A.S., &c. 
OFFER the following tables, with 
deference to the results of other 
observers, and likewise with great cau- 
tion, it being founded on an attempted 
measurement of the proportionate de- 
viation and relative quantity of the dif- 
ferently-coloured rays in the several 
stars stated in the table. And I request 
that allowance be made for the great 
difficulty of such observations ;. and that 
the noyelty of the experiments, the few 
opportunities that I have had of repéat- 
ing them on an extensive scale, toge- 
ther with the great nicety requisite 
where such small. quantities are .con- 
cerned, may apologize for the rude and 
imperfect manner in which the observa- 
tions have been conducted. 
If the future observations of more 
able persons should confirm my own, I 
shall be glad to see them in detail: if 
they should be confuted, the very detec- 
tion of the cause of the error may lead 
on to the knowledge of some other fact 
of use in the very curious investigations 
now going forward as to the nature of 
light.. At all events they have afforded 
me, and may afford to others, agreeable 
amusement during the long and dreary- 
winter nights of this miserable country. 
In the following table I assume the 
apparent altitude of the stars above the 
horizon at the time of observation to 
be 10°, and the barometer 0’ 760 :— 
the centigr. thermometer + 10°. Un- 
der which circumstances, we may esti- 
mate the mean refraction as 5’ 19”.10, 
according to the formula determined by 
La Place, and published in the Connais- 
sance des Temps. Now, presuming this 
to be the real mean refraction, and 
believing also that it equals the refrac- 
tion of Capella, the figures put down in 
column 3 of the following table, indi- 
cate the quantities of refraction to be 
added to, or subtracted from the above- 
mentioned mean, in order to get the 
_Monruty Mac. No. 400. 
true refraction of each of the stars 
named in-column 1]. Thus the stars 
which I have put above Capella in the 
column require a greater, and those 
below him a less correction for 10° of 
altitude than 5’ 19”.10, and probably in 
nearly the poportions indicated in the 
scale. The particular methods I have 
used, both for obtaining the spectra 
and for measuring the deviation of the 
extreme rays, being one, I believe, of 
my own discovery, and which I am not 
yet sufficiently confident in, 1 do not 
at present think it necessary to dis- 
close because I intend to make it the 
subject of a future communication. It 
has been overlooked, I .am_ persuaded, 
from its simplicity, in consequence of 
that fatal though common error of the 
human .understanding, whereby we so 
frequently dig deep for the discovery of 
objects that lie near the surface and are 
overlooked. All I desire is, to learn 
from those who have better means than 
I possess of making the experiments, 
whether astronomical observations ‘on 
those stars which are conveniently si- 
tuated for determining the refraction, 
shall be found to confirm or refute what 
appears to have been’ ascertained as 
probable from the employment ofa very: 
different mode of investigation. 
TABLE. 
Proper — 
Refraction at 
Names Apparent — pcre 
of Stars. | prevailing Colour, | the mean 
Refraction at 
5’ 19"'.10. 
Lyra... 2... Blue... ...... 6”.50 
Spica Blue ... 6.0 
Sirius. ....}| White J 5”.50 
Avided. Whitish ......} 4.0 
Atair. Whitish ...... 2”.50 
Procyon...| Yellowish white] 2”.20 
Rigel. ....] Yellowish white} 2”.0 
Capella. ..| Yellow.......} 0”.0 
Regulus... | Reddish white 1.0 
Arcturus ..| Orange Red...| 4.0 
Alpliard...] Reddish......| 4.50 
Betalgeus..| Red. ......-.] 4.50 
Aldebaran .} Reddish......| 7.0 
Observe.—In the above table the 
quantities put down are not asserted 
O with / 
