Here is Edmund Burke from the 18th century with the definitive guide:
(From his Sublime and Beautiful)
Obscurity.
To make anything very terrible, obscurity seems in general to be necessary. When we know the full extent of any danger, when we can accustom our eyes to it, a great deal of the apprehension vanishes. Every one will be sensible of this, who considers how greatly night adds to our dread, in all cases of danger, and how much the notions of ghosts and goblins, of which none can form clear ideas, affect minds which give credit to the popular tales concerning such sorts of beings. Those despotic governments which are founded on the passions of men, and principally upon the passion of fear, keep their chief as much as may be from the public eye. The policy has been the same in many cases of religion. Almost all the heathen temples were dark. Even in the barbarous temples of the Americans at this day, they keep their idol in a dark part of the hut, which is consecrated to his worship. For this purpose too the Druids performed all their ceremonies in the bosom of the darkest woods, and in the shade of the oldest and most spreading oaks. No person seems better to have understood the secret of heightening, or of setting terrible things, if I may use the expression, in their strongest light, by the force of a judicious obscurity than Milton. His description of death in the second book is admirably studied; it is astonishing with what a gloomy pomp, with what a significant and expressive uncertainty of strokes and coloring, he has finished the portrait of the king of terrors:
“The other shape,
If shape it might be called that shape had none
Distinguishable, in member, joint, or limb;
Or substance might be called that shadow seemed;
For each seemed either; black he stood as night;
Fierce as ten furies; terrible as hell;
And shook a deadly dart. What seemed his head
The likeness of a kingly crown had on.”
In this description all is dark, uncertain, confused, terrible, and sublime to the last degree.
Section IV.
Of the difference between clearness and obscurity with regard to the passions.
It is one thing to make an idea clear, and another to make it affecting to the imagination. If I make a drawing of a palace, or a temple, or a landscape, I present a very clear idea of those objects; but then (allowing for the effect of imitation which is something) my picture can at most affect only as the palace, temple, or landscape, would have affected in the reality. On the other hand, the most lively and spirited verbal description I can give raises a very obscure and imperfect idea of such objects; but then it is in my power to raise a stronger emotion by the description than I could do by the best painting.
And I think there are reasons in nature, why the obscure idea, when properly conveyed, should be more affecting than the clear. It is our ignorance of things that causes all our admiration, and chiefly excites our passions. Knowledge and acquaintance make the most striking causes affect but little. It is thus with the vulgar; and all men are as the vulgar in what they do not understand. The ideas of eternity, and infinity, are among the most affecting we have: and yet perhaps there is nothing of which we really understand so little, as of infinity and eternity. We do not anywhere meet a more sublime description than this justly-celebrated one of Milton, wherein he gives the portrait of Satan with a dignity so suitable to the subject:
“He above the rest
In shape and gesture proudly eminent
Stood like a tower; his form had yet not lost
All her original brightness, nor appeared
Less than archangel ruined, and th’ excess
Of glory obscured: as when the sun new risen
Looks through the horizontal misty air
Shorn of his beams; or from behind the moon
In dim eclipse disastrous twilight sheds
On half the nations; and with fear of change
Perplexes monarchs.”
Here is a very noble picture; and in what does this poetical picture consist? In images of a tower, an archangel, the sun rising through mists, or in an eclipse, the ruin of monarchs and the revolutions of kingdoms. The mind is hurried out of itself, by a crowd of great and confused images; which affect because they are crowded and confused. For separate them, and you lose much of the greatness; and join them, and you infallibly lose the clearness. The images raised by poetry are always of this obscure kind; though in general the effects of poetry are by no means to be attributed to the images it raises; which point we shall examine more at large hereafter. But painting, when we have allowed for the pleasure of imitation, can only affect simply by the images it presents; and even in painting, a judicious obscurity in some things contributes to the effect of the picture; because the images in painting are exactly similar to those in nature; and in nature, dark, confused, uncertain images have a greater power on the fancy to form the grander passions, than those have which are more clear and determinate. But where and when this observation may be applied to practice, and how far it shall be extended, will be better deduced from the nature of the subject, and from the occasion, than from any rules that can be given.
I am sensible that this idea has met with opposition, and is likely still to be rejected by several. But let it be considered that hardly anything can strike the mind with its greatness, which does not make some sort of approach towards infinity; which nothing can do whilst we are able to perceive its bounds; but to see an object distinctly, and to perceive its bounds, is one and the same thing. A clear idea is therefore another name for a little idea.