I'd like to comment on a subject I feel very strongly about at a quite personal level. I feel this issue is important, despite being overlooked by many serious authors of various backgrounds and heights. Quite frankly, it gets right on my tits.
It is this:
Anyway, as you may well know, the big problem is that in order to see, your retina has to absorb light (hence why it's black). But if you're invisible in the way described (i.e. everything that constitutes you/your body is completely transparent), light would pass through your retina, the photoreceptive cells that turn light into neural signals never would get stimulated, and you'd be as blind a completely blind person.
There are a few ways you could try and get around this. In the The Invisible Man, H. G. Wells attempts to do so by describing the recently-made-invisible protagonist's retinae** as being translucent in chapter 20:
Of course, this state of affairs would be no better than being blind. Without an optically correct cornea, lens and associated fluids that make your eyes so juicy and delicious, light would not get focused on your retina - the best you could hope for is a blur. And even if these weren't completely transparent and had their normal optical properties, without an opaque sclera (the white that surrounds the rest of your eye) light would be arriving at your retina from all other directions, meaning the world would be a big white blur. In order to see, you'd need these bare essentials to be unaffected by any invisibility remedy, but that would leave you with two very distinct and conspicuous white spheres bobbing around at everyone else's eye-level (unless you're very short/tall or you want to go around crouching).
As an aside, if you could really do this you may actually see better than you do ordinarily. The photoreceptors in your retina are orientated with their photoreceptive pigment at the back of your eye, meaning the cell bodies as well as various retinal processing cells that aggregate/mediate their responses are all in front of the receptors themselves, in the path of the light. I don't know how much it would actually improve your sight, but it wouldn't be by a vast amount - the best values of visual acuity that have been measured are not much better than the theoretical limit, given the distance between adjacent photoreceptors in the retina.
In conclusion, next time I'm watching a film with an invisible person in it, I'll just pretend they suddenly developed extraordinarily good echolocation.
* An attempt at reconciling this problem is made in the book - see later.
** This is a weird one - saying "retinae" instead of "retinas" sounds weird to me, but writing "retinas" instead of "retinae" looks equally weird. And anyway, both are acceptable and I like writing footnotes.
It is this:
An invisible person would be blind.This is, of course, ignoring any invisibility obtained through magic or similar, since then an author can also get around this problem with hand-waving and magic. I'm talking more about the sort of thing described in H. G. Wells' The Invisible Man*, or the modern film Hollow Man, where people are described as simply having all their matter turn completely transparent somehow. Fringe cases (such as the girl in The Incredibles) could be argued either way, I guess.
Anyway, as you may well know, the big problem is that in order to see, your retina has to absorb light (hence why it's black). But if you're invisible in the way described (i.e. everything that constitutes you/your body is completely transparent), light would pass through your retina, the photoreceptive cells that turn light into neural signals never would get stimulated, and you'd be as blind a completely blind person.
There are a few ways you could try and get around this. In the The Invisible Man, H. G. Wells attempts to do so by describing the recently-made-invisible protagonist's retinae** as being translucent in chapter 20:
an attenuated pigment still remained behind the retina of my eyes, fainter than mist.
Of course, this state of affairs would be no better than being blind. Without an optically correct cornea, lens and associated fluids that make your eyes so juicy and delicious, light would not get focused on your retina - the best you could hope for is a blur. And even if these weren't completely transparent and had their normal optical properties, without an opaque sclera (the white that surrounds the rest of your eye) light would be arriving at your retina from all other directions, meaning the world would be a big white blur. In order to see, you'd need these bare essentials to be unaffected by any invisibility remedy, but that would leave you with two very distinct and conspicuous white spheres bobbing around at everyone else's eye-level (unless you're very short/tall or you want to go around crouching).
As an aside, if you could really do this you may actually see better than you do ordinarily. The photoreceptors in your retina are orientated with their photoreceptive pigment at the back of your eye, meaning the cell bodies as well as various retinal processing cells that aggregate/mediate their responses are all in front of the receptors themselves, in the path of the light. I don't know how much it would actually improve your sight, but it wouldn't be by a vast amount - the best values of visual acuity that have been measured are not much better than the theoretical limit, given the distance between adjacent photoreceptors in the retina.
In conclusion, next time I'm watching a film with an invisible person in it, I'll just pretend they suddenly developed extraordinarily good echolocation.
* An attempt at reconciling this problem is made in the book - see later.
** This is a weird one - saying "retinae" instead of "retinas" sounds weird to me, but writing "retinas" instead of "retinae" looks equally weird. And anyway, both are acceptable and I like writing footnotes.
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