Confused about the meaning of a sentence from Joseph Conrad's The Secret Agent

The fol­low­ing ex­cerpt is from The Se­cret Agent, by Joseph Con­rad:

“Typ­i­cal of this form of de­gen­er­acy—these draw­ings, I mean.”

“You would call that lad a de­gen­er­ate, would you?” mum­bled Mr Ver­loc.

Com­rade Alexan­der Os­sipon—nick­named the Doc­tor, ex-med­i­cal stu­dent with­out a de­gree; af­ter­wards wan­der­ing lec­turer to work­ing-men’s as­so­ci­a­tions upon the so­cial­is­tic as­pects of hy­giene; au­thor of a pop­u­lar quasi-med­i­cal study (in the form of a cheap pam­phlet seized promptly by the po­lice) en­ti­tled “The Cor­rod­ing Vices of the Mid­dle Classes”; spe­cial del­e­gate of the more or less mys­te­ri­ous Red Com­mit­tee, to­gether with Karl Yundt and Michaelis for the work of lit­er­ary pro­pa­ganda—turned upon the ob­scure fa­mil­iar of at least two Em­bassies that glance of in­suf­fer­able, hope­lessly dense suf­fi­ciency which noth­ing but the fre­quen­ta­tion of sci­ence can give to the dul­ness of com­mon mor­tals.

“That’s what he may be called sci­en­tif­i­cally. Very good type too, al­to­gether, of that sort of de­gen­er­ate. It’s enough to glance at the lobes of his ears. If you read Lom­broso—”

I fail to un­der­stand the mean­ing of those words in bold; can any­body ex­plain or para­phrase the sen­tence with the bold part for me?

turned upon the ob­scure fa­mil­iar of at least two Em­bassies

The definition of familiar that is relevant here is "One who frequents a place." So I would expect that the person being referred to (looking at the book, this appears to be Mr Verloc) hangs around two embassies.

that glance of in­suf­fer­able, hope­lessly dense suf­fi­ciency

The appropriate definition of sufficiency, from the Lexico dictionary is

1.2 archaic Self-sufficiency or independence of character, especially of an arrogant or imperious sort.

which noth­ing but the fre­quen­ta­tion of sci­ence can give to the dul­ness of com­mon mor­tals.

Here, frequentation means that he studies science a great deal. I don't believe that this implies that he is particularly good at it ... there's some sarcasm here.

So "the Doctor" believes he is an authority on science and that he is talking to an ignoramus, and this causes him to look at Mr Verloc with an insufferably arrogant gaze.

Comrade Alexander Ossipon[…]—

turned upon = directed towards

{the obscure familiar of at least two Embassies} – the regular visitor to at least two Embassies of whom little was known

that glance – that particular facial expression

of insufferable, hopelessly dense sufficiency – that arrogantly indicated that he was tired of explaining why he had called the artist “degenerate” as the reason for this was blatantly obvious and tedious and was not worthy of responding to.

which nothing but – which [attitude] nothing except

the frequentation of science – the habitual involvement with science

can give = can add and a further attribute

to the dullness of common mortals. – to the stupidity of so-called ordinary people.