Early, amazing confirmation about xrays through glass:
From Richard Feynman - Wikipedia (no prime source given):
[…]at the Trinity bomb test. Feynman claimed to be the only person to see the explosion without the very dark glasses provided, reasoning that it was safe to look through a truck windshield, as it would screen out the harmful ultraviolet radiation.
Link to column:
From “Los Alamos From Below”:
“They gave out dark glasses that you could watch it with. Dark glasses! Twenty miles away, you couldn’t see a damn thing through dark glasses. So I figured the only thing that could really hurt your eyes - bright light can never hurt your eyes - is ultraviolet light. I got behind a truck windshield, because the ultraviolet can’t go through glass, so that would be safe, and so I could see the damn thing. OK.”
Which was reprinted as a chapter in “Surely You’re Joking Mr. Feynman.”
I don’t think this was all that smart because even a small percentage of UV getting thru the glass would be a large amount for an A-bomb. In addition, there was some uncertainity about the amount of different radiations that might be produced.
And note that X-rays and UV are quite different things.
I expect that Feynman was working off his own understanding of what radiation would be produced from the uranium fission reaction and how it would interact with the truck windshield. Which was probably relatively accurate. 
Wasn’t he blinded for a few minutes? I read this story a long time ago.
Of course it was a small bomb (10Kt?) and 20 miles away, so presumably the blast wouldn’t have been that effective. Probably no more than looking at the sun for a few seconds. Of course, this was a clculated gamble - nobody was sure what would happen - some were taking bets on the end of the world. (a pretty one-sided bet.)
Besides, what would dark glasses do - at least a truck window is about twice as thick. What’s dark glass made of, and how much UV would it really block? I asume it’s particles of something opaque suspended in glass, thereby blocking X% of whatever’s going through.
BTW, there’s enough of the less harmful UV through glass to (a)cause tans, as Cecil points out, and (b) to damage some materials over time. Many art galleries try to limit the amount of natural sunlight, as the UV will destroy some pigments. Of course, we’re talking decades and centuries of exposure…
Actually, just a few months of daily exposure to sunlight can wash out many pigments, particularly pigments primarily composed of organic molecules. (Pigments in which the primary colorant and binders are mineral tend to be generally more resistant to washout.)
As for the o.p.'s question, as has been pointed out, it is ultraviolet light (to which the 21% oxygen/78% nitrogen atmosphere is relatively transparent) that is of concern. The atmosphere almost completely opaque to x-rays at the distances at which observing a nuclear explosion without becoming a victim of it is concerned; the x-rays are actually absorbed by the atmosphere and turn into the thermal pulse and pressure waves. The amount of UV at the distance they were at would not do any permanent damage to the retina, although it might cause temporary flash-blindness.
Stranger
The link is in my post.
BTW, I forgot to mention that X-rays easily pass thru glass. Otherwise Röntgen would have never discovered them coming out of his glass tube. But that’s neither here not there wrt Feyman’s tale.