Magnifying Glass Lamp Lens Ring

These are sold in many office supply stores, Walmart, and Amazon.
Mines changed slightly but here's the current version
http://www.officedepot.com/a/products/729355/Ledu-Full-Spectrum-Magnifier-Desk-Lamp/

The magnifying glass is held in by a thin walled, lipped bushing. If you tend to grab and twist on the front of the lamp, like most people, you can crack the ring. Then the glass falls out. I tried gluing it (Go E6000!!) but it soon cracked again.

Here's a replacement ring....it should