, 
DISPERSION. . 
diferent ees pied from the different ee of the 
media 3 an that the refrangibiit Ke the ex~ 
if 
<i 
i ferent media were alike, the angles of difperfion in thofe 
ia would likewife be alike 3 but it was referved for Mr, 
Dolcnd to thew the fallacy of this fuppofition, si was 
confidered as true by all the philofophers of bis tir Mr. 
Doilond began to furpeét the truth of it, and in ee year 
1757 he commenced a courfe of laborious experiments which 
confirmed him in his fufpicion, gradually fhewed him that dif- 
rity. Ries D ollond a: thus fhewn the important ufes 
which enters a dark room ee eane a hole, a then to mea- 
fure, as exactly as poffible, the angle of refraction, and the 
breadth of the fpeétrum upon a sags placed at a given 
diltance from the prifm ; 3 an thefe apr the 
angle o ion 
angle - - age in proportion B aera 
may be deduced ; thus, by making prif{ms of eat 
fabhances, exadly on the fame refractive angle, and pla- 
cing them fucccflively in the hke circumitances, the peculiar 
difperfive powers of thofe fubftances may be afcertained. 
re Euler, who gives ap bees ar alee re{pecting’ the 
pease of this operation, advifes to make ufe of prifms 
ing angles, not lefs than 70 degrees. 
02, art. xii. 
e of e 
fractive and difperfive powers ‘@ means of eer aap 
r. W. H. Wollaften ; but as the principal ufe of this 
ho ai is ae pena the refra€tive powers of fubftances, 
the reader wil 1 under the article Rerracrion. 
With athe to othe difperfive powers, Dr. Wollatton did 
not meafure them with accuracy, nor indeed does his method, 
without fome farther improvement, feem capable of deter 
mining thefe powers wit precifion. We hal 
all in 
Taste of the cue -Powers of various tranfparent 
Subitances, in comparifon . the difperiive Power of 
Water, which is el TO 
Difperfion, 
White flint-glafs, of the {pecific gravity 3.29, 180 
M. 3, F.1, 709 
Glafs made of minium; wiz. a 1; ot 
I, x, 482 
red-lead, and flint, in the 3 ; 325 
. . > > 
proportion o hid 7 266 
} 200 
_ Common piate-glafs, or aa aes fp. Br. 4 76, 165 
Crown-glafs, fp. gr. 2 : 148 
Brazil pebble, fp. 2.6 159 
Glais tinged red by means a gold, fot enamel, 290 
Difperfion. 
Glafs of Saint oe in Brances . » 149 
iam - . 286 
Rock-crytt al, - _ 121 
Another fpecimen oe the fame, - - 124 
yeah ee ), ~ ° 16g 
other fpecimen a ue Cae : - 233 
Wat ter, - - fete) 
bees faturated with common falt, 122 
sa of fal annenes or of raurated am- 
134 
Nitric aca - - - - 154 
Dr. Wollafton’s Table of difperfive Powers, refpecting 
which he fays, 
sé T have e eaiauiee to reduce the feveral fubftances, 
thus examined, to one table ; but as the iimits of colour are 
in few inftances fafliciently well defined ad accurate me 
furation, I have not attempted to a numerical atk 
mate of their powers, but have merely ena the or 
der in which they foceed each other 
In this order the fubftances now follow : 
Sulphur. Amber, 
Giafs of net (4 ace Diamond, 
Balfam fF Tolu u, 
Oil of faffatras, 
Muriate of antimony, 
Plate-glaf, Dutch, 
Plate-glafs, Engliflr, 
Guaiacum, Crown-glafs 
Oil of cloves, Ruby, (onlay 
Flint-glafs, Water, 
Colophony, ee acid, 
Canada balfam, 
Oil of amber, Sulphate of barytes, 
Ja Selenite, 
. Jargon, 
Oil of turpentine, 
| 
Copal, 
Balfam of Capivi, 
White fapphire, 
Anime, luor {par 
Iceland {par, 
Mr. Zeiher of ee who compofed the fix kinds 
of glafs made with flint and m 
minium, end examined their 
difperfive eae Mwhich 2 are ftated at the commencement of 
the firtt table, willing to give a {uperior or more ufeful con- 
fiftence to thofe giaffes, tried to mix alkaline falts with 
them; but he was much furprifed to find that this mixture 
greatly diminifhed the mean refraGion, almof without 
making any change in the eee ae After many trials, 
of glafs, greatly fuperior 
ruction of telefcopes, 
reat a difperfioa of the rays as comm 
time the mean aa was only as 1.61 t 
Berl. 1766.) Having now fated all the moft effeutial ob. 
fervations which have been made’ philo- 
ional to en refrative power, the phenomenon would occa- 
3; but fince there feems to 
fatisfaftory information abou 
tious philofophers have not farnithed any. me more tha 
3 njeClures. 
