. respecting Douhh Refraction. 1S9 
To explain the unusual refraction of Iceland crystal by 
pression or motion propagated, has not hitherto been attempted 
(to my knowledge) except by Huygens, who for that end sup- 
posed two several vibrating mediums within that crystal. But 
when he tried the refractions in two successive pieces of that 
crystal, and found them such as is mentioned above, he con- 
fessed himself at a loss for explaining them. For pressions or 
motions, propagated from a shining body through an uniform 
medium, must be on all sides alike ; whereas by those exper;- 
ments it appears, that the rays of light have different properties 
in their different sides. He suspected that the pulses of aether 
in passing through the first crystal might receive certain new mo- 
difications, which might determine them to be propagated ip 
this or that medium within the second crystal, according to the 
position of that crystal. But what modifications those might be 
he could not say, nor think of any thing satisfactory in that 
point And if he had known that the unpspal refraction de- 
pends, not on new modifications, but on tjie priginal and un. 
changeable dispositions of the rays, he would have found it as 
difficult to explain how those dispositions which he supposed to 
be impressed on the rays by the first crystal, could be in them 
before their incidence on. that crystal; and in general, how all 
rays emitted by shining bodies, can have those dispositions in 
them from the beginning. To me, at least, , this seems inexpli- 
cable, if light be nothing else than pression or motion propagated 
through aether.” 
Observations on Sir Isaac Newton's Rule of Double 
Rfraction, 
Those who have already examined the Law of Double Re- 
fraction, as given by Huygens, and its agreement with observa- 
vations' made in all sections of the crystal of Iceland-spar, must 
experience no small degree of surprise, when they find that Sir 
Isaac Newton has proposed another law, different from his^ 
and absolutely incompatible with observation. As Sir Isaac 
remarks that Huygens has described the phenomena more 
* “ Mais pour dire comment cela se fait, je n’ay rien trouve jusqu’ ici quj 
satisfasse.” — C. Huygens, De la Lumiere^ c. v. p, 91. 
VOL. TV. NO. 7. JANUARY 182L I 
