200 Newton's Letters, Hypothesis and Experiments 



if a refracted ray (as n L) be made incident, the incident 

 (A m) shall become the refracted, and therefore if the ray 

 A j«,v, after it is arrived at v, where I suppose it parallel to the 

 refracting superficies, should be reflected perpendicularly 

 back, it would return back in the line of incidence vpA; 

 therefore going forward, it must go forward in such another 

 line v 7r A, both cases being alike, and so be reflected at an 

 angle equal to that of incidence. 



This may be the cause and manner of reflexion, when light 

 tends from the rarer towards the denser aether ; but to know 

 how it should be reflected when it tends from the denser to- 

 wards the rarer, you are farther to consider, how fluids near 

 their superficies are less pliant and yielding than in their more 

 inward parts, and if formed into thin plates or shells, they be- 

 come much more stiff and tenacious than otherwise- Thus 

 things which readily fall in water, if let fall upon a bubble of 

 water, they do not easily break through it, but are apt to slide 

 down by the sides of it, if they be not too big and heavy. So 

 if two well-polished convex glasses, ground on very large 

 spheres, be laid one upon the other, the air between them 

 easily recedes till they almost touch, but then begins to resist 

 so much that the weight of the upper glass is too little to bring 

 them together, so as to make the black (mentioned in the other 

 papers I send you) appear in the midst of the rings of colours. 

 And if the glasses be plain, though no broader than a two- 

 pence, a man with his whole strength is not able to press all 

 the air out from between them, so as to make them fully touch. 

 You may observe also that insects will walk upon water with- 

 out wetting their feet, and the water bearing them up ; also 

 motes falling upon water will often lie long upon it without 

 being wetted. And so I suppose aether in the confine of two 

 mediums is less pliant and yielding than in other places, and 

 so much the less pliant by how much the mediums differ more 

 in density ; so that in passing out of denser aether into rarer, 

 when there remains but a very little of the denser aether to be 

 passed through, a ray finds more than ordinary difficulty to 

 get through, and so great difficulty where the mediums are of 

 a very differing density as to be reflected by incurvation after 

 the manner described above, the parts of aether on the side 

 where they are less pliant and yielding, acting upon the ray 

 much after the manner that they would do were they denser 

 there than on the other side ; for the resistance of the medium 

 ought to have the same effect on the ray from whatsoever cause 

 it arises. And this I suppose may be the cause of the reflexion 

 of quicksilver and other metalline bodies. It must also concur 

 to increase the reflective virtue of the superficies when rays 



