What's the meaning of "by" in here?

[dilodi83]

What's the meaning of "by" in this sentence?

- That small window doesn't let in enough light to read by.

Is "by" part of the verb? I cannot catch its meaning.

[dilodi83]

***** NOT A TEACHER *****

I understand it to mean 'It's too dark to read by the window because it doesn't let enough light in'.

I also caught this meaning, but I cannot get "by". Do you know if the phrasal verb "read by" exist? What's the function of this preposition in this sentence?

[hoangkha]

What's the meaning of "by" in this sentence?

- That small window doesn't let in enough light to read by (that light).

Is "by" part of the verb? I cannot catch its meaning.

Regularly reading by the light of a candle is easy to be short-sighted.
[not a teacher]

[Chicken Sandwich]

***** NOT A TEACHER *****

Sorry, dilodi83, I deleted my reply because, upon reflection, I realised that my rewording didn't capture the meaning of your sentence and consequently didn't answer your question. I think that hoangkha explanation is spot-on - reading by the light of an object to me just means 'using' that light for reading. You could be reading by the light of the moon without being anywhere near the moon.