OPT 
tic ; and M. Huygens further modified the Cartefian liy- 
pothefis, imagining the waves of light to fpread, not cir¬ 
cularly, but in ellipfes ; and upon this fuppofition he en¬ 
deavoured to explain the phenomena of Iceland cryftal. 
Kircher was the firft who obferved the remarkable pro¬ 
perties of the infulion of lignum nephriticum (Hyperan- 
thera), the optical phenomena of which have been the 
fubjeCl of various difcufiion from his time to that of fir 
Ifaac Newton, who firft explained them fatisfaClorily. 
In the fubfequent period, feveral philofophical focieties 
were formed, which very much contributed to the im¬ 
provement of optics, as well as other fciences. The per- 
fons who diltinguiihed themfelves at this time were, Dr. 
Hooke, fecretary to the Royal Society, Grimaldi in Italy, 
De la Hire, M. Marriotte, M. Perrault, among the French, 
and particularly M. Huygens, of whofe treatifeon Dioptrics 
Newton always profelfed a very high opinion. Otho 
Guericke and father Cherubini alfo delerve mention. 
The greateft difcovery that was made during the period 
to which we have now advanced in the hiftory of optics, 
was the velocity with which light is tranfmitted. Galileo 
attempted to meafure it, but failed ; the Academy del Ci- 
mento purfued the fubjeCt according to the mode pro- 
pofed by Galileo : need we be furprifed at their want of 
fuccefs ? At length the objeCl was accomplifhed by 
Roemer. See the article Light, vol. xii. p. 675. 
The Hon. Mr. Robert Boyle made a variety of experi¬ 
ments in order to difcover the firft principles and caufes 
of colours, and the conftitution of bodies on which they 
depend ; but the enumeration of them would require a 
detail for which we have not room. The faCts concern¬ 
ing lignum nephriticum , fuggefted by Kircher, were fully 
afcertained by Mr. Boyle, who alfo corrected feveral of 
the hafty obfervations of Kircher concerning the colours 
that appear in the infufion of this fubltance. In this 
tinCture he obferved the difference between reflected and 
tranfmitted light. The principal phenomena of this in¬ 
fufion are eafily explained by the Newtonian doCtrine of 
the different refrangibility and reflexibility of the rays of 
light, and the difpolition of particular bodies to refleCt 
fome kinds of rays, and to tranfmit others ; fo that, if the 
reflected light fall upon the eye, they fhall appear to be of 
one colour, when, by the tranfmitted light, they appear to 
be of another. The firft diftinCt account of the colours ex¬ 
hibited by thin plates, of various fubftances, occurs among 
the obfervations of Mr. Boyle. The fubjeCt was purfued 
by Dr. Hooke; and he was the firft to obferve, if not to 
deferibe, the beautiful colours that appear in thin plates 
of Mufcovy-g'iafs. Mr. Haukfbee, about this time, made 
many experiments by means of a prifm for afeertaining 
the refraCtive powers of different fluids ; and he drew up 
a table that exhibited their fpecific gravities, the angles at 
which they were obferved, and the ratio of refraCtion. 
Dr. Hooke firft fuggefted the idea ef making allowance 
for the effect of the refraCtion of light in palling from the 
higher and rarer, to the lower and denfer, region of the 
atmofphere, in the computed height of mountains ; and 
thus he accounts for the difference among authors with 
regard to the height of feveral very high hills. 
But, of all the difeoveries that have been made at any 
time concerning the nature of light and colours, thofe of 
fir Ifaac Newton are pre-eminent; and they are comprifed 
in his Optics. The chief of them were communicated to 
the Royal Society, feven or eight years after they were 
made, in a letter to the fecretary, Mr. Oldenburg, dated 
Cambridge, Jan. 18, 1672. 
The Englifh have reafon to boaft of their labours on 
the fubjeCt of Optics. Befides the dife-overies of the im¬ 
mortal Newton, the LeCtures of Barrow, and the works 
of Smith and Harris, give us a pre-eminence, in this 
branch of fcience, above the mathematicians of the con¬ 
tinent. The Optics of Smith, although defective in plan 
and method, is ftill a very valuable performance; and 
even now the forms of his demonftrations are followed as 
the molt Ample and evident. Excepting Euler’s work on 
ICS. 547 
Optics, who, in his extenfive walk, vifited'every fpot of 
fcience, we do not recoiled! any complete and fyfteinatic 
publication by the foreign mathematicians ; and, indeed, 
the treatile of Euler is not perfect ; but one of the vo¬ 
lumes of d’Alembert’s Opufcules is devoted entirely to 
optical refearches; and the TranfaCtions of the Societies 
of Paris and Berlin, contain many valuable memoirs on 
this fubjeCt. Among Englilh books, we mult not omit 
Dr. Prieftley’s elaborate “ Hiftory of the prefent State of 
Difeoveries relating to Vifion, Light, and Colours, 4-to. 
1772;” and Dr. Wood’s “ Elements of Optics, defigned 
for the Ufe of Students in the Univerfity of Cambridge,” 
8vo. 3d. edit. 1811. to which laft excellent work we lhalL 
be greatly indebted in the progrefs of this article. 
PRINCIPLES OF OPTICS. 
Difeoveries concerning the Refrattion of Light. 
Though the ancients made few optical experiments, 
they neverthelefs knew, that, when light palled through 
media of different denfities, it did not move in a ftraight 
line, but was bent ov ref ratted out of its original diredtion. 
This was probably fuggefted to them by the appearance- 
of a ftraight rod partly immerfed in water; and, ac¬ 
cordingly, we find many queltions concerning this and 
other optical appearances in the works of Ariftotle. Ar¬ 
chimedes is laid to have written a treatile on the appear¬ 
ance of a ring or circle under water; and, therefore, could 
not have been ignorant of the common phenomena of re¬ 
fraCtion. The ancients, however, were not only ac¬ 
quainted with thefe more ordinary appearances, but alfo 
with the production of colours by refraction. Seneca fays,, 
that, if the light of the fun Ihines through an angular 
piece of glafs, it will Ihow all the colours of the rain¬ 
bow. Thefe colours, he fays, are falfe, Inch as are feen 
in a pigeon’s neck when it changes its pofition ; and of 
the fame nature, he fays, isafpeculum, which, without 
having any colour of its own, aflumes that of any other 
body. It appears alfo, that the ancients were not igno¬ 
rant of the magnifying power of glafs globes filled with 
water, though they do not feem to have been acquainted 
with its caufe ; and the ancient engravers are fuppofed to 
have ufed a glafs globe filled with water to magnify their 
figures. This indeed feems evident, from their lenticular 
and fpherical gems of rock-cryital which are ftill pre- 
ferved, the effeCt of which, in magnifying at lead, could 
fcarcely have efcaped the notice of thofe who had often 
occafion to handle them ; if indeed, in the fpherical or 
lenticular form, they were not folely intended for the 
purpofes of burning. One of thefe, of the fpherical kind,, 
of about an inch and a half diameter, is preferved among 
the foflils prefented by Dr. Woodward to the univerfity 
of Cambridge. 
The firft aftronomers were not aware that the interval# 
between liars appear lefs near the horizon than near the 
meridian ; but it is evident that Ptolemy was aware of 
this circumftance, by the caution which he gives to allow 
fomething for it, upon every recourfe to ancient obferva¬ 
tions. Ptolemy alfo advances a very lenlible hypothehs,. 
to account for the greater apparent fize of the fun and 
moon, when feen near the horizon. The mind, he fays,, 
judges of the fize of objeCts by means of a preconceived 
idea of their dillance from us : and this diftance is fan¬ 
cied to be greater when a number of objeCts intervene 
which is the cafe when we fee the heavenly bodies near 
the horizon. In his Almageft, however, he afcribes 
this appearance to a refraction of the rays by vapours, 
which actually enlarge the angle fubtended by the lumi¬ 
naries. 
The nature of refraCtion was afterwards confidered by 
Alhazen, an Arabian writer; infomuch that, having made 
experiments upon it at the common lurface between air> 
and water, air and glafs, water and glafs, and being 
prepoflefled with .the ancient opinion of cryltalline orbs 
in the regions above the atmofphere, he even fufpeCted % 
x refraction 
