We have been discussing the use of full sentence definitions to begin a general-specific passage. A relative clause is usually needed in this case.
Let’s look at the following example from our textbook [1]:
A microscope is an optical instrument with which the apparent size of an object can be enhanced.
A question was raised as if we can write the sentence as:
A microscope is an optical instrument, with which the apparent size of an object can be enhanced.
The answer is NO. Think for a second why?
The original sentence is a restrictive relative clause, but the modified one is non-restrictive. After adding a comma, we actually alter the scientific meaning behind. To clarify the difference between a restrictive and a non-restrictive relative clause, let’s see the simple example below.
1. My brother, who is living in Baltimore, is an engineer.
2. My brother who is living in Baltimore is an engineer.
The first sentence is a non-restrictive clause, meaning that I have only one brother and by using “who is living in Baltimore” I am just providing additional information to the brother. This information does not restrict which bother I am referring to, since I only have one. Note that a pair of commas is a symbol of parenthetical expression, meaning that the information within the commas is additional, not necessary.
The second sentence is a restrictive clause, meaning that I have multiple brothers and by using “who is living in Baltimore” I am providing necessary information as which brother I am referring to. The meaning behind is that only the one living in Baltimore is an engineer, and my other brothers live elsewhere are not engineers.
Now let’s look back at the first sentence. If we add a comma before the clause, we are saying that all optical instruments can enhance the apparent size of an object, which is not true. Note that the restrictive clause restricts the range of optical instrument we are referring to. Here we are saying a microscope is such a subset of optical instruments that are able to enhance the apparent size of an object.
Reference:
[1] John M. Swales & Christine B. Feak, Academic Writing for Graduate Students: Essential Tasks and Skills, University of Michigan Press
