On my first playthrough of Syndicate, and I have a question regarding the Lydia memory

I just finished the first mission in it, and thought that was the whole thing, especially after the Juno cutscene, but was surprised when I was thrown back into that world. the early and mid 20th century are some of my favourite time periods to study as a historian, so this is a really cool inclusion for me, and don't want to leave it only for it to be lost forever until I choose to replay it, but I noticed a portal that says "Portal to 1868", and I'm weary to enter it in fear of not being able to come back, but I don't know for certain and Google hasn't given me a straight answer either as to whether you can return after completing the memory, so I figured I'd turn to Reddit... after completing the Lydia memory are you free to return whenever you'd like? Or is it a one-and-done situation? Please let me know!