But there are many interesting facts connected with our vision which may be plainly described. Some people are obliged to carry an object (or a book) to within ten inches of the eye to see it distinctly; and a person who does not possess convergent power of the eye will have to move it farther off, or use convex glasses; while a “near-sighted” person, whose eyes are too quickly convergent, will use concave glasses to bring the object near to the eye. There is but one small place in the retina of the eye which admits of perfect vision. This, the most sensitive portion, is called the “yellow spot,” and vision becomes more and more indistinct from this point towards the circumference. This can be proved by any one; for in reading we are obliged to carry our eyes from word to word, and backwards and forwards along the lines of print. Another very important element in our vision is the contraction and enlargement of the iris around the pupil. In cases where strong light would only dazzle us the iris expands, and the pupil is contracted to a sufficient size to accommodate our vision. At night, or in a darkened room, the pupil is enlarged. This change will account for our not being immediately able to see objects when we have passed from darkness to great light, or _vice versâ_. The iris must have time to accommodate itself to the light. Now, outside the small space of perfect vision above mentioned, there is a circle of considerable extent, called the “field of vision.” In man this field, when the eyes are fixed, subtends an angle of about 180°, because beyond that the rays cannot enter the pupil of the eye. But in the lower animals, the fish and birds,—notably the ostrich,—the field of vision is much more extensive, because the pupils are more prominent, or the eyes are set more towards the sides of the head. The ostrich can see behind him, and fish can see in any direction without apparent limit. Man can only see indistinctly; and though he can move his eyes rapidly, he can see distinctly but a small portion of any object at a time, yet he sees with both eyes simultaneously a single object, because the two lines of vision unite at a single point, and as the two images cover each other we perceive only one image. Beyond or within this point of meeting the vision is indistinct, but the angle of convergence is always varied according to the distance of the object. If we hold up a penholder in front of us, and in a line with any other defined object,—say an ink-bottle,—we can see the penholder distinctly, and the ink-bottle indistinctly, as _two_ images. If we then look at the ink-bottle we shall see it single, while the penholder will appear double, but with imperfect outlines. [Illustration: Fig. 147.—The Stereoscope.] [Illustration: Fig. 148.—Mode of taking photograph for Stereoscope.]