of Light ; and a new Theory of Vision. 421 



surface while the drop and the leaf are separate, it is evident 

 that there must be a substance between, and that it is this sub- 

 stance, and not the water, that reflects this light. 



Persons under water and looking upwards towards the surface 

 will behold the images of objects that are below, reflected down- 

 wards from above. Now if water had the property of reflecting 

 light back attain into the medium, all the particles throughout 

 would reflect light as well as those that are upon its surface, for 

 all have the same nature, and then there could be no distinct 

 image of any thing, but only a mass of light; and, by the same 

 rule, the power of reflecting light does not belong to the air, for 

 we can only account for the distinctness of the image by sup- 

 posing that this power is confined to the particles that are im- 

 mediately on the surface of the water ; and no reason can be given 

 why those particles should reflect light any more than the othei-s 

 that are above, except by supposing that they undergo some che- 

 mical change or decomposition by coming in contact with the 

 water from which the others are excluded; and in this case it 

 could not be the air itself, but some fluid or gas which makes a 

 part of the composition of air, that has the property of reflecting 

 light; and this is all that I am anxious to maintain- 

 That there is a similar substance adhering to the surfaces of 

 gla«s which has the property of reflecting light, may be proved 

 beyond the power of a dispute, because it may be removed by 

 the pressure of the finger, and when removed there is no light 

 reflected from that part of the surface. Glass reflects light from 

 both its surfaces; and whenever we look into glass, of any figure*, 

 we may behold two distinct images of the same objects reflected 

 from the two opposite sur-taces. I have now in my hand a plano- 

 convex lens, and on pressing my finger hard upon the spot where 

 the image of any object is reflected from the further surface of 

 it, so much of the image is removetl as corresponds with the lines 

 of my finger, whilst in the interstices between the lines, where 

 there can be no pressure, the light is reflected as strong as ever. 



Now, if the light from these objects passes through the glass 

 before it is reflected, it cannot possibly be the glass that reflects 

 it back again; and if it had not passed through, the pressure of 

 my finger could not in any way produce the effect I have de- 

 scribed. It is evident then that it is not the glass that reflects light 

 inwardly, whatever it may do outwardly; and if what I have al- 

 ready said is not sufficient to satisfy the reader that it is not the air 

 that has this property, Sir Isaac Newton has proved it past all 

 •ontroversy, by showing that glass reflects light from this surface 



• \Vlien the surfaces are parallel to each other, ns in pUiiii glass, the two 

 images enter the eye in the same tingle, and then they can only he distin- 

 guished by holding the glass obliquely. 



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