To achieve this upside down lens effect

The next step in the evolution of the date window magnifier was very clever, and almost seems obvious given the result. Watch makers took the magnifier lens and placed it upside down on the bottom of the crystal, as opposed to on the top. By reversing the lens, to magnify in this new direction, the same effect is achieved without the bump on top of the crystal. You can see this feature on some of the more expensive watches. A few notable ones are the Zenith Defy series, the Louis Vuitton Tambour line, and the Chopard Mille Miglia line of watch models. This upside down magnifier lens feature is not limited to these models, and should be found on a number of newer watches. As time goes on, these new types of lenses are sure to appear on less expensive models. To achieve this upside down lens effect, there must be enough clearance between the face of the watch, and the crystal. With the trend in watches being larger in size, this is often not an issue, and the upside down lens is actually a bit smaller than would be needed on top of the crystal. Needless to say, this feature is very clever, and works wonderfully in providing the best of both worlds to those of us who want a clean looking watch that is visually convenient.