Something like a cross between an expensive shirt and a telephone message

Already before his death, Proust must have been anticipating the way 'une espèce d'instrument optique' would be mistranslated in Enright's revised version of the Scott Moncrieff and Kilmartin as 'a sort of magnifying glass', because in the letter in reply to André Lang, which was published in Les Annales only months before he died, he is at pains to explain that he prefers the use of a telescope to the microscope as an analogy of what he is doing in his novel, where he is 'trying to discover universal laws' rather than analysing himself 'in the personal and odious sense of the word'.

Proust wants to emphasise the distance between the writer and the object that he pursues in his writing. As he continues in this letter:

It has to do with drawing a reality out of the unconscious in such a way as to make it enter into the realm of the intellect, while trying to preserve its life, not to garble it, to subject it to the least possible shrinkage -- a reality which the light of intellect alone would be enough to destroy, so it seems. To succeed in this work of salvage, all the forces of the mind and even of the body, are not superfluous. It is a little like the cautious, docile, intrepid effort necessary to someone who, while still asleep, would like to explore his sleep with his mind without this intervention leading to his awakening. Here precautions must be taken. But although it apparently embodies a contradiction, this form of work is not impossible.

Unfortunately I don't have the original French for this letter. The English is from the 1950 translation by Mina Curtiss. In Ronald Hayman's biography of Proust, the translation he cites (which might be his own) is:

It is a matter of drawing something out of the unconscious to make it enter the domain of consciousness, while trying to preserve its life, [not to] mutilate it, to keep leakage to a minimum -- a reality which could apparently be destroyed by exposure to the light of mere intelligence. To succeed in this work of salvage, the whole strength of the body and the mind is not too much. Something like the same kind of effort -- careful, gentle, daring --  is necessary to someone who while still asleep would like to examine his sleep with his intelligence, without letting this interference wake him up.

I like the way, in Hayman's translation, his use of the words 'mutilate' and 'leakage' evoke the delicate, membranous anatomy of some unknown submarine creature; Curtiss's 'shrinkage' and 'garble' deaden the image, turning it into something like a cross between an expensive shirt and a telephone message.