ECHO chambers and prisms. The first is a social media chamber where people gather to hear their own opinions bounced back. The second is a partial lens showing one view of a situation.

Whether the Labour Party does or doesn’t have a problem with antisemitism seems to be a hall of mirrors moment. Only this time there’s an angry crowd in there shouting and waving placards.

On one level, this is one of those social media tales where someone comments on something they shouldn’t have commented on or shares something better left unshared.

The arc of this story, or one arc of this story, goes like this: in 2012, Labour leader Jeremy Corbyn put a comment on a Facebook post from a Los Angeles-based street artist called Mear One. The artist had painted a mural in London featuring anti-Semitic images and this was due to be removed after complaints.

The artist said: “Tomorrow they want to buff my mural Freedom of Expression. London Calling, Public art.”

According to reports, Corbyn replied: “Why? You are in good company. Rockerfella [sic] destroyed Diega Viera’s [sic] mural because in included a picture of Lenin.”

Here we can pause in the hall of mirrors and look at ourselves frowning. A quick Google helps. Turns out Corbyn was referring to a fresco by Diega Rivera in New York City’s Rockefeller Center – painted in 1933 and then disowned by the Rockefeller family, who objected to the inclusion of an image of Lenin.

Here is where those prisms come in. Corbyn seems to have dusted off a very old prism to dredge up this political parallel. And he was so busy peering through that prism that he forgot to look closely at the offensive image he was accidentally defending.

There it might have ended, if not for the persistence of one man, a music publicist called Sam Shemtob.

According to this morning’s Guardian, Mr Shemtob complained on social media and then pursued the Labour Party for an official apology for Corbyn’s apparent defence of the antisemitic street art. He didn’t get one, and that’s how a moment on Facebook has landed Corbyn and Labour with such a suppurating problem.

Only of course it’s more complicated than that, as suggested by yesterday’s rally outside Parliament. Members of London’s Jewish community protested about Labour’s antisemitism problem – alongside rival Jewish groups, some reportedly carrying banners proclaiming: “Jews for Jez.”

As someone who tilts to the left but doesn’t belong to the Labour Party, all of this puzzles me. Does Labour really have this problem, beyond potty old Ken Livingstone ranting about Hitler believing in Zionism?

Well, it has enough of a problem to set up the Chakrabarti report of 2016 into whether it had a problem or not. This report concluded that: “The Labour Party is not overrun by antisemitism”.

A relief – but not enough to stop worries, with further examples having emerged, including Bradford West MP Naz Shah sharing something offensive on Facebook. Corbyn suspended her for that offence.

Is this partly the prism thing? It is common for people on the left to condemn the behaviour of Israel. Does that reflex instinct somehow turn into antisemitism?

I don’t know the answer to that, but you do have to be careful with those prisms. Jeremy Corbyn’s reluctance to join the criticism of Russia over the Wiltshire poisonings seems to be down to an old Soviet-era disinclination to criticise Russia.

Corbyn needs to be wary of smudged old prisms. And he needs to stamp out this antisemitism problem, or the perception of a problem, before the stain spreads.