QUOTE(critic @ Mar 10 2005, 03:07 PM)
I'm still not sure if there is such a difference.
Neither am I.
QUOTE
What was the purpose of using techeilet as a siman? Because it is something so small and the difference so hard to distinguish, that it had to absolutely be day in order for someone to in fact see the difference.
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I think you're assuming that its talking about looking at one's tzitzis. But its probably talking about bundles of dyed wool. At least R. Eliezer must be talking about that, rather than 7 green strings and one blue string. That would make no sense. So presumably whatever R. Eliezer is referring to finds its parallel in the white and blue wool. In other words, its not talking about tzitzis. Also, the Gemara makes it clear that it is poorly dyed wool with different streaks in it. Its hard to see how that could refer to a tzitizis string dyed techeles.
In any case, I can definitely hear that "techeles" is not so specific, rather it means a shade of blue like techeles. Of course in the Mishna when they had techeles they would have simply said "techeles". Maybe its not so specific.
On the other hand, maybe it is. Maybe its talking of the specific hue called techeles and only when we
have techeles can we be certain what that hue is, and thus use it for a simman when to recite shema.