negative investigation

The two words not ... either form a 'construction' which can be used when something is not the case and you wish to indicate that something else is not the case.

Peter: I don't know why the sky is blue
John: I don't [=do not] know either.

I don't know as well. - not used
I don't know too - not used.

You can signify an agreement of a negative thing with 'too' by putting it first:

Peter: I don't know why the sky is blue
John: I, too, don't know.

Thank you~

But is this admissible?:

--I don't know also. (=I don't know either)

No.

When using the fixed construction 'not... verb... either', the word 'not' goes before the verb, and 'either' goes after the verb.

John does not know the time. Peter does not know either.
Mary cannot swim. Jane cannot either.
My father does not know where his hat is. I do not either.

Or you can use 'neither' and a verb following.

John does not know the time. Neither does Peter.
Mary cannot swim. Neither can Jane.
My father does not know where his hat is. Neither do I.

But when using 'too', and 'also', these go before the verb.

This is less usual.

John does not know the time. Peter, too/also, does not know.
Mary cannot swim. Jane. too/also cannot.
My father does not know where his hat is. I too/also do not.

Thank you~

Me neither.

Sunlight (of all colours) is scattered by air molecules; blue is scattered more because of its shorter wavelength. I learned this when I was 7, courtesy of Arthur Mee's Children's Encyclopaedia (1909 edition) which I had found at a school jumble sale.

I knew that (I think). I was just introducing the commonly-heard, if slightly ungrammatical, expression.

I think negative adjectives could be used with EITHER, though more common with TOO; do you agree?:

-This theory is impossible either.
-I am uninterested in this either.

Though TOO is possible.

You cannot use 'either' like this. We use 'either' after 'not' e.g. John is not here either.

You can use 'too' or 'also' after words like impossible and uninterested.

To clarify, to express that a negative quality or state is held by anther person or thing, we can use 'not ... (positive adjective)... 'either', like this:

John is not brave either.
My car is not reliable either.
You are not wise either.

But if we use adjectives of the 'un-'. 'in-', im-' etc type to express the same idea, we use also, too, as well, etc, just as if the adjective was a praising one.

John is uncourageous also.
My car is unreliable too.
You are unwise as well.

Thank you~