The sentence I wrote is as follows:
The idea behind this technique was that the human eye could not differentiate between the individual colors of each point, thereby causing the eye to blend the colors together to form a more complete image.
My professor said that the "thereby" in that sentence should have a semicolon behind it and a comma in front, but as far as I know, a semicolon separates two independent clauses. I don't think "causing the eye to blend colors together …" is a complete sentence, but she said it should be written as ";thereby,". Is she correct?
Best Answer
If I had to use the exact words that you used, I would punctuate the sentence exactly as you have: with a comma before thereby and no punctuation immediately after it.
I suspect, however, that the professor's suggested alteration may have been motivated by dissatisfaction with the wording of the sentence, and by the erroneous idea that a change in punctuation would take care of the sentence's perceived shortcomings.
To me, the awkwardness of the wording is due primarily to its association of thereby (which means essentially "in this way" or "by this means") not with an action but with a negative capability (the inability of the human eye to differentiate between the individual colors of each point). To avoid that awkward association of ideas, I would remove thereby and reword the sentence as follows:
The comma between that and because is optional (depending on the approach you take toward punctuation); the comma between point and the is necessary.