DIOPTRICS, 
angle of difpertion vCr,i is the n direQion of the re 
fra&ted light, and me is its fine, which is called a of the 
mean angle of refraiion: vf, and rd, e the fines 
tremes; vf being called the fine of mo ae 
lour, viz. of the violet, and r d, the fine of the leaft refrangible 
colour, which is the red. is feparation of the w 
colourlefs light, into various colours, jultly induced fir Ifaac 
Newton to conclude, that white light c confitts, or is a mix- 
ture, of differently coloured rays, which, being differently 
ee are of courfe a by the refracting me- 
n the angle of inciderce, 
of reft ation, is conliderably 
ngle of diffipation will be fo large as to ex- 
In order to render this coloure 
viv Hen edhe above-men mp 
nent eer rays of white hehe have kei baa ella 
the fat Seed colours. See Paismatic Colou 
refraGtive mediums have on difperfive 
oe “os inftance, the angle of incidence » fig. 6 
remaining the fame, not only the mean angle of refraction 
m C FE will vary according as the refraGtive medium DG 
3s water, or glafs, or oil, or other tra ajay fubftance ; but 
refraing mediums the me 
whilft 7 angle of aera is {maller. 
mediums the mean angie of refraction is {maller, whilit the 
angle o difperfion is larger. In fhort the knowledge = as 
mean refractive power of any given fubftance, will n 
able us to determine the precife quantity ss its slpeive 
pow vice verfae erefore, in each particu 
tranfparent fubttance, both its refraCtive a. its cpaie 
properties muff be afcertained iy means of actual oes 
nts. 
tiple refractive pov 
lift of the ‘itpevige powers of various tranfparent a 
ftances will be found annexed to the li — refraGtive pow 
under the articles RerractTion, and PERSION, to ‘which 
we refer our readers. e fhall, howerer a add a few 
ufeful and general bane on refraction 
Upon n the whole it appears, that the denier bodies; viz. 
the raye of light, 
eepting (as fir (ae Newton expreffea it) that unCluous and 
oe eee refra& ely than others of the fame den- 
tT ce that has an intermediate re- 
in water, or diftiiled water, 
fo) 
2s 
o 
=> 
= 
other known fubitance, and to this property its pecuhar bril- - 
liancy, when properly fhaped, is in great meafure to be attri- 
uted. Soiaucusiaier. have a greater refractive power in 
proportion to their ftrength. Spirit of turpentine is the 
moit pal amongtt the fluids. 
above-mentioned properties of tranfparent bodies, -or 
of light, or of both, however trifling they may appear tothe 
novice in philofophy, have proved extremely ufcful to the 
human fpecies. Sir Ifaac Newton, who firit paid particular 
attention, and made feveral important difcoveries relative to 
the coloured rays of white light, had no fufpicion of the 
laft above-mentioned property of tranf{parent fubftances. He 
; thought that the difperfive power was conftantly proportion= _ 
ate to the refradlive. Experiments made“fubfequent to his, 
have fhewn, that the faé& is otherwife ; fo much - ia even 
glaffes a of different materials, or ay the fam 
ceived a moft capital improvement ; the n 
be briefly pointed out in this place, while the detail of par- 
ee wil be found under the articles AcHRromarTic, and 
eal lens, fuch as is ufed for telefcopes, bends the rays 
of light towards a particular point or {mall fpot, which is 
called its focus ; but this bending of the rays of light is ac- 
mpan 
companied with a decompofition of the fame into their co- 
loured component rays; therefore, the magnifying powers of 
the lenfes of the old telelcopes were always attended with 
tin the various prifmatic urs, w ecame m 
apparent in proportion as the magnifying power was great- 
der iven length of telefcope; bence thofe tele- 
fcopes could not to magnify the obj: & beyond a 
ode gree; nor could their objet lenfes have a 
fhould coun. 
bending the rays of light to a focus, and at the fame time 
not decompofe them into their coloured parts. And from 
his effet the achromatic lenfes, or the achromatic ae 
fcopes, have derived their diftinGtive appellation. This for 
of telefcope, then, has a vaft advantage over thofe of the old 
conttruétion ; for fince the effet of its obje& lens is not 
tinged with colours, the power of the telefeope may be in- 
creafed to a confiderable degree, and its objeét lens may be 
rifms, of telefcopes with lenfes, ee various other op 
tical inftruments, which chdoibrealy belong to the dioptric 
branch of optics, but it has already been mentioned, that 
thefe will be found defcribed under ae articles of their par= 
ticular AMES. 
Diopraic 
