It was Hume's object to push his scepticism to its most extreme verge. Thus pushed, it "so wrought upon" him that he was "ready to reject all belief and reasoning" till a return to every-day life made his speculations appear in his own eyes "cold and strained and ridiculous." What then was the inference Hume himself intended? Which was really groundless—every-day belief or scepticism? Will his useful dilemma induce the reader to receive Kant's excuse for the celebrated doubter, when he bids us let the man alone because he is but trying the strength of human reason? At all events, Hume's way of stating his case seems to justify the old remark, that, while Superstition is refuted by Reason, Nature itself refutes the Sceptic.
B.—EXTRACTS FROM POPULAR LECTURES, BY PROFESSOR HELMHOLTZ, ON THE RECENT PROGRESS OF THE THEORY OF VISION.
"If now we compare the eye with other optical instruments, we observe the advantage it has over them in its very large field of vision. This for each eye separately is 160° (nearly two right angles) laterally, and 120° vertically, and for both together somewhat more than two right angles from right to left. The field of view of instruments made by art is usually very small, and becomes smaller with the increased size of the image.
"But we must also admit, that we are accustomed to expect in these instruments complete precision of the image in its entire extent, while it is only necessary for the image on the retina to be exact over a very small surface, namely, that of the yellow spot. The diameter of the central pit corresponds in the field of vision to an angular magnitude which can be covered by the nail of one's forefinger when the hand is stretched out as far as possible. In this small part of the field our power of vision is so accurate that it can distinguish the distance between two points, of only one minute angular magnitude, i.e. a distance equal to the sixtieth part of the diameter of the finger-nail. This distance corresponds to the width of one of the cones of the retina. All the other parts of the retinal image are seen imperfectly, and the more so the nearer to the limit of the retina they fall. So that the image which we receive by the eye is like a picture, minutely and elaborately finished in the centre, but only roughly sketched in at the borders. But although at each instant we only see a very small part of the field of vision accurately, we see this in combination with what surrounds it, and enough of this outer and larger part of the field, to notice any striking object, and particularly any change that takes place in it. All of this is unattainable in a telescope.
"But if the objects are too small, we cannot discern them at all with the greater part of the retina.
'When, lost in boundless blue on high,
The lark pours forth his thrilling song,'
the 'ethereal minstrel' is lost until we can bring her image to a focus upon the central pit of our retina. Then only are we able to see her.