Yes, you can access many, but not all, online resources this way.
To access resources only available via SIA log-in, you have two options: follow a link provided by the library (e.g. there is a link in the databases guide, or follow this link to Google Scholar); or
- on the Scholar homepage, choose Settings from the top menu, and then Library links from the menu on the left. Enter Sotheby's in the search box that appears, and you should be given the option to tick and then save Sotheby's Institue of Art - New York/UK. Your choice should be saved until you clear your cookies.
If an article or other document is available via SIA's subscriptions, you will see a link "online @ SIA" on the right of the Google results screen.