This section is from the book "Trout Fishing", by W. Earl Hodgson. Also available from Amazon: Trout fishing..
Trout's Eyesight Very Keen—Deer, Grouse, and Wild Duck Rely on Other Senses—Do Trout Hear? — Misapprehensions About Light—Mr. Disraeli, P-P-A-, and Others—How Things Look from Under the Water—Emotional Illusions Leading to Misunderstandings—"Old John's" Amusing Statement—Light Important Only as a Symptom—Adverse Conditions—The Ideal Morning.
It is generally taken for granted that the light in angling is a highly important consideration. The assumption is reasonable. Fish differ from all other game in respect that in relation to the sportsman only one of their senses, that of sight, seems to be of service.
Deer, for example, are at a much greater advantage. Besides seeing with their own eyes, they are quick to perceive danger through the conduct of other animals. If sheep in their neighbourhood are disturbed, the deer know that man is near, and are alert, probably bolting, in a moment. They are quick of hearing, too. If one may judge from the silence which the stalker imposes even when far off, a man's footfall a mile away may be as audible to them as it would be to an Australasian Black listening with his ear to the ground. Above all, they have a sense of smell extraordinarily acute. If you are to stalk a stag successfully, you must from the very start, which may be three miles out of range, keep to lee of him, which, as the air takes strange turns among the mountains, is no easy matter. A blunder on the part of the sportsman will enable the stag to scent danger at an incredible distance; and then, in a double sense, the game is up.
Similarly, grouse not only see quickly : evidently they have sharp ears as well. Excepting on the few moorlands that are still almost of primitive wildness, they are nowadays driven towards the guns from the opening of the season. An attempt to shoot them over dogs would not lead to satisfactory results. The birds would usually rise beyond the distance within which it is sportsmanlike to shoot.
Then, wild duck: Who that has watched the habits of these attractive birds, a couple of which would so conveniently fill the space which at the end of the day is often vacant in one's trout-creel, can question that they are almost preter-naturally equipped for the battle of life ? There are hundreds on or in the immediate neighbourhood of the lake by the side of which these words are written; but they are practically as safe as they would be if they were on the carefully guarded waters of St. James's Park. If one lay out all night, armed, among the reeds at the head of the lake, where two streams run in, a brace might be taken from the flight of duck that are often seen there in the morning; also, it might be possible, at any time of day, to stalk the wild fowl in a slowly-moving boat with a screen of bushes in the bow; but the most cunning attempt to get at them in a candid way would be a failure. Wherever they may be resting, their position is always such that they are forewarned of your approach from the front, or from the rear, or on either flank.
The trout are in quite different case. They seem not to hear. At any rate, if they do hear, they are never, so far as one can judge, disturbed by noise. They show no sign of alarm when a railway train rushes over a bridge above the stream in which they are lying, or rising; often they are equally unconcerned amid the loudest peals of thunder. They must, it is true, have the sense of smell. Only on that assumption is it possible to account for their taking a worm, or a gentle, or a piece of roe, or the grub of a wasp, or that of a stone-fly, in flood water too thick to be seen through; but their sense of smell seems to be only a guidance to their food, not a sense through which they are warned of the approach of foes. They never fly from a man until they see him. For safety against their enemies, that is to say, trout practically depend upon their eyes alone. After they are hooked, their strength, and the instinct that leads a few of them to run into weeds or other cover, may be of use; but their eyes are their primary and main defence. It is reasonable to assume, then, that their eyes are sharper than those of most creatures.
That being so, it is reasonable that when we go fishing we should be anxious about the light. What is wanted, it is commonly supposed, is a light that will blot out the rough edges of the tackle, soften down any excess of gaudiness in the flies, and make the lures look natural.
What is this light? The answers by any dozen anglers, even if they were men of much experience, would be of striking variety. One would say that a dull day is the best. Perhaps that would be the general opinion. It is noticeable that Mr. Disraeli and other novelists who are careful about local colour usually have the sky well clouded when hero or heroine, or both, set out to fish by the banks of some romantic stream. Each of the rest of our dozen witnesses might have a theory of his own. As a rule it would be a negative theory. "A glare on the water" would be the bane of one ; another would like a thin veil of fleecy clouds; another would prefer the light of a day, characteristic of April, on which the sun is hidden and peeps out alternately ; another would have but little hope if the ripples were tipped with silvery gleams ; another would dread "lanes of light" lying upon the surface of the water ; others, according to individual fancies, would think well of any light in which the water was not too blue, or too gray, or too yellow, or too red, or too green, or too purple. Probably the only thought on which all would be unanimous is that the light which falls from a cloudless sky would never do at all. It is generally supposed that good sport is not to be had in unmitigated sunshine.
At first it may seem presumptuous on the part of a single fisherman to question the opinions of all these twelve gentlemen ; but it is not really so. If all the twelve were of the same mind, the single fisherman might be considered arrogant; but, as each of the twelve is assumed to have a theory differing from every one of the others, the criticism is merely a modest contribution of the thirteenth.
 
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