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the Theory of Light and Colours. 
“ and is performed only in the passage of the ray by the body, 
“ and at a very small distance from it. So soon as the ray is 
“ past the body, it goes right on.” (Optics, Qu. 28.) 
Now the proposition quoted from the Principia does not di- 
rectly contradict this proposition ; for it does not assert that 
such a motion must diverge equally in all directions; neither 
can it with truth be maintained, that the parts of an elastic me- 
dium communicating any motion, must propagate that motion 
equally in all directions. (Phil. Trans, for 1800. p. 109 iiz<) 
All that can be inferred by reasoning is, that the marginal 
parts of the undulation must be somewhat weakened, and that 
there must be a faint divergence in every direction ; but whe- 
ther either of these effects might be of sufficient magnitude to 
be sensible, could not have been inferred from argument, if the 
affirmative had not been rendered probable by experiment. 
As to the analogy with other fluids, the most natural inference 
from it is this : “ The waves of the air, wherein sounds consist, 
« bend manifestly, though not so much as the waves of water 
water being an inelastic, and air a moderately elastic medium ; 
but ether being most highly elastic, its waves bend very far less 
than those of the air, - and therefore almost imperceptibly. 
Sounds are propagated through crooked passages, because their 
sides are capable of reflecting sound, just as light would be pro- 
pagated through a bent tube, if perfectly polished within. 
The light of a star is by far too weak to produce, by its faint 
divergence, any visible illumination of the margin of a planet 
eclipsing it ; and the interception of the sun's light by the moon, 
is as foreign to the question, as the statement of inflection is 
inaccurate. 
To the argument adduced by Huygens, in favour of the 
