Q: Migrated photo library

In my pictures folder I've got two 'packages' named:

iPhoto Library.migratedphotolibrary; (36.07GB) and

Photos Library.photoslibrary (20.37GB)

I'm guessing the second is the package used by the new Photos application, but what is the first? Do I need it? Can I archive it (on a separate medium)? I'm concerned that the second is quite a bit smaller than the first, suggesting that not everything was migrated. Is there a simple way to check?

iMac (21.5-inch, Late 2009); Yosemite 10.10.3; Photos Version 1.0 (209.52.0); 3.06 GHz Intel Core 2 Duo; 16 GB 1067 MHz DDR3; NVIDIA GeForce 9400 256 MB