[Footnote: The diagram belonging to this passage is placed between lines 5 and 6; it is No. 4 on Pl. VI. ]
A guiding rule.
225.
OF THE ATMOSPHERE THAT INTERPOSES BETWEEN THE EYE AND VISIBLE OBJECTS.
An object will appear more or less distinct at the same distance, in proportion as the atmosphere existing between the eye and that object is more or less clear. Hence, as I know that the greater or less quantity of the air that lies between the eye and the object makes the outlines of that object more or less indistinct, you must diminish the definiteness of outline of those objects in proportion to their increasing distance from the eye of the spectator.
An experiment.
226.
When I was once in a place on the sea, at an equal distance from the shore and the mountains, the distance from the shore looked
much greater than that from the mountains. On indistinctness at short distances (227-231).
227.
If you place an opaque object in front of your eye at a distance of four fingers' breadth, if it is smaller than the space between the two eyes it will not interfere with your seeing any thing that may be beyond it. No object situated beyond another object seen by the eye can be concealed by this [nearer] object if it is smaller than the space from eye to eye.
228.
The eye cannot take in a luminous angle which is too close to it.
229.
That part of a surface will be better lighted on which the light falls at the greater angle. And that part, on which the shadow falls at
the greatest angle, will receive from those rays least of the benefit of the light.
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230.
OF THE EYE.
The edges of an object placed in front of the pupil of the eye will be less distinct in proportion as they are closer to the eye. This is shown by the edge of the object n placed in front of the pupil d; in looking at this edge the pupil also sees all the space a c which is beyond the edge; and the images the eye receives from that space are mingled with the images of the edge, so that one image confuses the other, and this confusion hinders the pupil from distinguishing the edge.
231.
The outlines of objects will be least clear when they are nearest to the eye, and therefore remoter outlines will be clearer. Among objects which are smaller than the pupil of the eye those will be less distinct which are nearer to the eye.
On indistinctness at great distances (232-234).
232.
Objects near to the eye will appear larger than those at a distance.
Objects seen with two eyes will appear rounder than if they are seen with only one. Objects seen between light and shadow will show the most relief.
233.
OF PAINTING.
Our true perception of an object diminishes in proportion as its size is diminished by distance.
234.
PERSPECTIVE.
Why objects seen at a distance appear large to the eye and in the image on the vertical plane they appear small.
PERSPECTIVE.
I ask how far away the eye can discern a non-luminous body, as, for instance, a mountain. It will be very plainly visible if the sun is behind it; and could be seen at a greater or less distance according to the sun's place in the sky.
[Footnote: The clue to the solution of this problem (lines 1-3) is given in lines 4-6, No. 232. Objects seen with both eyes appear solid since they are seen from two distinct points of sight separated by the distance between the eyes, but this solidity cannot be represented in a flat drawing. Compare No. 535.]
The importance of light and shade in the perspective of disappearance (235-239).
235.
An opaque body seen in a line in which the light falls will reveal no prominences to the eye. For instance, let a be the solid body and c the light; c m and c n will be the lines of incidence of the light, that is to say the lines which transmit the light to the object a. The eye being at the point b, I say that since the light c falls on the whole part m n the portions in relief on that side will all be illuminated. Hence the eye placed at c cannot see any light and shade and, not seeing it, every portion will appear of the same tone, therefore the relief in the prominent or rounded parts will not be visible.
236.
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OF PAINTING.
When you represent in your work shadows which you can only discern with difficulty, and of which you cannot distinguish the edges so that you apprehend them confusedly, you must not make them sharp or definite lest your work should have a wooden effect.
237.
OF PAINTING.
You will observe in drawing that among the shadows some are of undistinguishable gradation and form, as is shown in the 3rd [proposition] which says: Rounded surfaces display as many degrees of light and shade as there are varieties of brightness and darkness reflected from the surrounding objects.
238.
OF LIGHT AND SHADE.
You who draw from nature, look (carefully) at the extent, the degree, and the form of the lights and shadows on each muscle; and in
their position lengthwise observe towards which muscle the axis of the central line is directed.
239.
An object which is [so brilliantly illuminated as to be] almost as bright as light will be visible at a greater distance, and of larger apparent size than is natural to objects so remote.
The effect of light or dark backgrounds on the apparent size of objects (240-250).
240.
A shadow will appear dark in proportion to the brilliancy of the light surrounding it and conversely it will be less conspicuous where it is seen against a darker background.
241.
OF ORDINARY PERSPECTIVE.
An object of equal breadth and colour throughout, seen against a background of various colours will appear unequal in breadth. And if an object of equal breadth throughout, but of various colours, is seen against a background of uniform colour, that object
will appear of various breadth. And the more the colours of the background or of the object seen against the ground vary, the
greater will the apparent variations in the breadth be though the objects seen against the ground be of equal breadth [throughout].
242.
A dark object seen against a bright background will appear smaller than it is.
A light object will look larger when it is seen against a background darker than itself.
243.
OF LIGHT.
A luminous body when obscured by a dense atmosphere will appear smaller; as may be seen by the moon or sun veiled by mists. OF LIGHT.
Of several luminous bodies of equal size and brilliancy and at an equal distance, that will look the largest which is surrounded by the
darkest background.
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OF LIGHT.