A few more thoughts- I also like Independent's suggestion but it may not have to be split 50-50. Is it possible that your wife will be happy if just $X,000 (or $XX,000) is in a cash equivalent investment? It may be that she'd be happy with something less than half the assets in cash, but enough to make her feel secure that the two of you will be OK in a downturn.
Second- you may want to occasionally share with her the results on what you've invested- the ups AND the downs. Over the long run, she may become more comfortable with downturns after seeing that the good stuff rebounds eventually. Finally, you SHOULD share with her anything especially risky you might want to do. My late husband was a dear man with modest spending habits and a high IQ, but zero interest in investing. He was still my common-sense test. If he didn't like the idea of something I was planning to do, I either didn't do it or took a lower stake, because the outcome affected him, too. A friend once told me her husband the doctor lost half their retirement savings (they were in their 40s) by buying on margin just before the dotcom bubble. She had no idea that was what he was doing till it was too late.