> 
to ae ae 
OPTICS. 
from water in this refpect, nor the cryftalline from glafs, and 
oi the vitreous humour is a medium between them both. 
He alfo traced the progrefs of the poh of light through 
difcufling every hypo- 
onftr ates that it is 
other. ticl 
other cbfevations ef this ingenious philofopher that pertain 
F edes is faid effe@ed with 
urning mirrors, in deftroying the fleet of Marcellus; and 
obferving that th al diftance of concave {peculums was 
pofe, the ftory was difcredited, 
a fable, particularly by Des Cartes. Kir- 
os Pe pupil Schottus, however, thought the fubje& 
e particular inveftigation, efpeciaily as Pro- 
els i is Yai te shave deftroyed a fleet at psp by the 
ame means. ircher, with a view of terminating the dif- 
a 
mined to try the effect of many plane mirrors. 
he ere€ted a frame, on which he placed five of thefe mirrors, 
of ie fame fize, and fo difpofed as to throw the rays that 
were reflected from them, upon the fame place, at the dif- 
tance of more than 100 feet ; and thefe five mirrors produced 
fo great a degree of heat, that he had no doubt but that, 
by multiplying them, he could have fet fire to inflammable 
fubftances at a greater diftance. A drawing of this os 
bodies, as Ariftotle Sonccived but motion of a fubtle 
fluid, communicated inftantaneoully by the preflure of a 
Inftead of the perfectly folid globules, in 
which Des Cartes thought that light con- 
filted, Makbenche fubftituted fluid vortices, su fuppofed 
that every impreffion communicated to any one of them is 
immediately tranf{mitted to thofe that are eontgucs to it, fo 
that the propagation of light is fimilar to that of found. 
The later Cartefians in general fuppofed, that the fluid b 
which light is tran{mitted ts elattic, and M. Huy ygens further 
pei chan the dire@, the colour, he a is red ; if the 
aii be quicker, it is blue ; ; but if they be equal, it is yel- 
w 3; and out of thele three colours it was long the opinion 
=f many philofophers, that all the reft were formed, by dif- 
ferent proportions in their mixture. a neers this 
embarraffment and error with refpe&t to the nature of light, 
and colours, he juftly aidingaiies between black and white ; 
obferving that the former fuffoc 
erning ive 
of any chee near er it was placed, except ae that 
were white or red ; ound that there was no grounds for 
the exception of thefe two colours ; this power te Se 
to the imagination of the animal, as it lofes it when it is dead. 
Kircher was the firft who obferved the dpe properties 
of the infution of lignum nephriticum, — 
mena of which have been the fubje@ o 
from his ime to that of fir {aac Newton, w 
them fatisfactorily. J 
phic 
rault, amon 
m 
sar ae by Roemer. (See L 
r. Boyle made a variety of Sunes in order to difcover 
le “firtt principles and caufes of colours, and the conftitution 
of bodies on which they depend; but the enumeration = 
them would require a detail, for which we have not roo 
The facts concerning lignum nephriticum, fuggefted by Kir. 
cher, were fully afcertained by Mr. Boyle, who alfo cor- 
re&ted feveral of the hafty obfervations of Kircher concerns 
ing the colours that appear in the infufion of this fubftance. 
In this ture he obferved the diffrence between refleCted 
and tran{mitted light. The principal phenomena of this 
infufion are cafily explained by the Newtonian do@trine of the 
different refrangibility and ‘rellesibility of the rays of light, 
and the difpofition of particular bodies to reticct fome kinds 
of rays and to — others; fo that if the reflected light 
fall upon the eye, fhall appear to be of one colour, 
when by the ala et light they appear to be of another. 
The firft diftin@ account of the colours exhibited by thin 
plates, of various fubftances, occurs among the obiervations 
of Mr. Boyle. The fubjeét was purfued by Dr. Hooke ; 
and he was the firft to obferve, if not to defcribe, the beau- 
means of a prifm for afcertaining the refractive powers 
different fluids ; and he drew up a table that exhibited tee 
{pecific gravities, the angles at which they were obferved, 
and the ratio of refraétion. Dr. Hooke frit fuggelted the 
idea of aren allowance for the effect of the refraction o 
light in pafling 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. 
For an account of the difcovery of the infledion of light we 
refer 
