In order to search the Mumford Collection you need to conduct an "Advanced Search." From the left column of the search screen, select "Advanced Search."
In the Location menu, choose Special Coll. / Mumford
Continue by typing in a keyword or keywords into the search boxes. The example below uses the keyword "irrational."
The search of the Mumford Collection using the keyword "irrational," yields the following result, one the the titles that your professor recommends: