Coleridge wrote this luminously gloomy line not once but twice in his Notebooks: first in November 1799 [1:528] and then again, almost identically save for the capitalisation of ‘Gloom’ (and an extra comma) in October 1803 [1:1558]. Critics have been, by and large, rather struck by it. Elliott B. Gose, Jr. [‘Coleridge and the Luminous Gloom: An Analysis of the “Symbolical Language” in The Rime of the Ancient Mariner’, PMLA 75:3 (1960), 238-44] thinks the phrase unlocks the mysteries of the Rime. More proportionately, James Vigus, in his powerful study Platonic Coleridge (Leeds: LEGENDA 2009), sees the line as indicative of ‘Coleridge’s characterization of Plato as a mystic’:
The polarity of nonsense and truth, ‘dark with excessive bright’ is reflected in the [luminous gloom] image, which was so adhesive to Coleridge’s mind that he uses identical expressions in Notebook entries separated by years. [Vigus, 21]Vigus stresses that if STC thought Plato wrote ‘nonsense’ it was at least ‘dear gorgeous nonsense, worth unravelling’; and he points out that the November 1799 notebook entry is followed by the following bit of drafted (unpublished) Coleridgean verse:
Mist as from a volcano—Vigus goes on to pick out several related images from Coleridge's writing: a description of the religious mystic Jacob Boehme as one who ‘contemplated Truth and the forms of Nature thro’ a luminous Mist, the vaporous darkness rising from his Ignorance and accidental peculiarities of fancy and sensation, but the Light streaming into it from his inmost Soul’ and the ‘fair luminous mist’ of Dejection; an Ode. [Vigus, 22], amongst others. We could add many other examples. In 1802, for instance, he records in that Statius's nec caret umbra Deo (Thebiad 4:425) is ‘obscure indeed, but certainly profound’. The Latin means: ‘nor even the shadows lack Divinity’ [Notebooks 1:1179].
Waterfall rolled after long looking at like a segment of a Wheel
—the rock gleaming thro’ it—
Amid the roar a noise as of innumerable grasshoppers or of spinning wheels [Notebooks 1:529]
It seems nobody has noted that ‘luminous gloom’ is not Coleridge's coinage. He is quoting Dionysius the Areopagite, or as scholars now insist on calling him, pseudo-Dionysius the Areopagite (a much cooler name, I think: as if he wasn't a human being who happened to be called Dionysius like various other famous men, but was some kind of synthetic man or android). Anyway: Dionysius's Περὶ μυστικῆς θεολογίας (‘On Mystical Theology’, which probably dates to the early 6th century, but which Coleridge may have thought was a 1st-century work) ponders how we can view God, since God Himself is ‘viewless’, or invisible. The work argues that we approach God's brilliance and light through a mystic darkness, and that the mystery of the Trinity abides in a ὑπέρ-φωτον γνόφον, a ‘hyper-luminous gloom’. This little phrase is a famous bit of pseudo-Dionysius's work, actually (it occurs at the beginning of the opening prayer of the On Mystical Theology) and fairly widely known.
It's possible that Coleridge wasn't actually reading an edition of Dionysius at this time, but he was certainly reading the ninth-century theological writer John Scotus Erigena (in July 1803 STC wrote to Southey telling him ‘I have received great delight & instruction from Scotus Erigena’, and he often copied out bits of Scotus's Latin into his notebooks at this time) and Scotus very often quotes Dionysius—indeed, Scotus's contemporary fame, in an age when few scholars knew Greek, derived from the fact that he translated Dionysius and wrote a commentary upon him. The key thing is that Scotus used Dionysius to revive a neo-Platonic mode of Christian theology that was to go on to have a vast influence across medieval Europe. An 1803 Coleridgean notebook entry quotes Jonannes Scotus quoting Dionysius:
ipse omnium essentia est qui solus vere est, ut ait Dionysius Ariopagita. Esse, inquit, omnium superesse Divinitatis. ‘[He] who alone is the essence of things, truly is, as Dionysius the Areopagite put it: Being, he says, in all things [comes from] the over-being Divinity’ [Notebooks 1:1369]Here's my working theory, for which I need a little more (or, you know: any, at all) evidence: over the years 1799-1803 Coleridge was planning a poem, perhaps to be called ‘On Mystical Theology’, in which he versified and explored some of these Dionysian and Scotus-Ergenan neo-Platonic ideas in poetry. The line at the head of this post, provided we take luminous as a disyllable (‘lum’nous’), is a feminine-ending pentameter, after all. So, it might be that the concrete imagery of a poem like 1802's ‘Hymn before Sunrise, in the Vale of Chamouni’ (partly plagiarised from German poets though that poem was) might have provided the launch-point for some Neo-Platonic philosophical-poetizing. What is Mont Blanc, in that poem, if not luminously gloomy?
Deep is the air and dark, substantial, black,It's the divine inversion of Milton's ‘Darkness Visible’—‘Visibility Engloomed’ perhaps. Vigus quotes Ernst Cassirer (‘to poeticize philosophy and to philosophize poetry—such was the highest aim of all romantic thinkers’) before qualifying:
An ebon mass: methinks thou piercest it,
As with a wedge! But when I look again,
It is thine own calm home, thy crystal shrine,
Thy habitation from eternity!
O dread and silent Mount! I gazed upon thee,
Till thou, still present to the bodily sense,
Didst vanish from my thought: entranced in prayer
I worshipped the Invisible alone.
Those English and German Romantics who pursued this aim, including Coleridge, often looked naturally to Plato as a precedent or model. Nevertheless, a contrary model loomed larger still: Kant, who in an essay of 1796 attacked contemporary poetic, Platonizing philosophers as merely decking out their presumptuous, ungrounded assertion. ‘Philosophy is fundamentally prosaic,’ insisted Kant, who pointed to Plato as this vain tendency in philosophical writing. [Vigus, 6]‘This censure,’ Vigus adds, ‘is consistent with the restriction Kant places on speculative Reason. Unlike Coleridge (and much of Plato) Kant denies that human Reason can have access to the noumenal realm.’ Coleridge, in point of fact, was caught between his desire to see poetry's luminous gloom shine darkly through the noumenal truths of God, and his belief that he might, if he tried to capture this, do nothing more than throw chaff in the readers' eyes. Hence, perhaps, his repeated badgering of Wordsworth to write the great philosophical poem of the age: simultaneously an expression of his belief that such a poem could be written, and an admission that he didn't believe he could write such a poem. It's even possible he considered using this the sunny mist, the lum'nous gloom of Plato line as part of ‘Hymn before Sunrise, in the Vale of Chamouni’, which is certainly a poem as much about the mystic interpenetration of light and shadow, or blindness and vision, of materiality and spirit, as it is about anything. It might explain why that poem breaks off so abruptly.