Identifying The Poisonous Glass!

What was the challenge?

Let's number the 4 glasses as 1,2,3 and 4.

Here are steps to identify the glass with poisonous liquid in it. 

1. Empty the glass 1 into another empty glass.

2. Take about 1/4th liquid from glass 2 and pour it into emptied glass 1.

3. Take 3/4th of liquid from glass 3 and pout it into glass 1.

Now, glass 1 has 1/4th of liquid from glass 2 and 3/4th from glass 3.

4. Now put glass 1 and glass 4 on the measuring scale.

    4.1 - If it weighs exactly 500 gm then it suggests that glass 1 had poison.
  
    4.2 - If it weighs between 500 - 505 grams (precisely 502.5 if exactly 1/4th of liquid was taken from glass 2 to glass 1) then obviously glass 2 is with poisonous liquid.

    4.3 - If it weighs between 505 - 510 grams (precisely 507.5 if exactly 3/4th of liquid was taken from glass 3 to glass 1) then obviously glass 3 is with poisonous liquid.

    4.4 - If it weighs exactly 510 gm then it suggests that glass 4 has poison.