They are used by jewellers, watchmakers, engravers, and others who examine small objects. What are microscopes? They are instruments used for examining very small objects. How does a drop of rain-water or vinegar appear under the microscope? It appears to be full of strange-looking creatures, which are always in motion. How do the smallest insects appear? They appear to be as perfectly formed as the larger ones which are seen with the naked eye. What may be seen with a microscope? Thousands of things too small to be seen by us unless they are magnified. What kind of spectacles do near-sighted people use? Near-sighted people use glasses, thickest at the edge and thinnest at the centre. What kind of spectacles do we use as we grow old? As we grow old we use glasses, thinnest at the edge and thickest at the centre. Of what use are spectacles? When the vision is defective, spectacles enable us to see objects more clearly. How should spectacles be made? They should be so made as to render objects distinct, but neither to magnify nor diminish them. As people grow old, how do their eyes change? Their eyes change so that they see objects at a distance better than those which are nearer, hence, in reading they hold a book farther from the eye. The Eye. What is that part of the eye called, which is blue, gray, or brown, in different persons? It is called the _iris_. What is the pupil of the eye? The pupil is the black spot or hole in the centre of the iris. [Illustration: Fig. 11.] What is the use of the pupil? The pupil is the window of the eye, and the rays of light which enter the eye must pass through it. What power has the iris over the pupil? The iris has the power of making the pupil larger or smaller, according to the amount of light which it receives. How does a bright light affect the iris? It causes the iris to contract, so that the pupil becomes smaller. How does a faint light affect the iris? It causes the iris to dilate, so that the pupil becomes larger. Of what advantage is the iris to the eye? The iris acts like a sentinel to protect the eye from any sudden light. How does a sudden light affect the eye? A sudden light causes pain to the eye. Of what shape is the pupil of the eye? In man, the pupil of the eye is circular. What is the size of the pupil of the eye? The pupil varies from the one-twentieth to the one-third of an inch in diameter, depending upon the brightness of the light entering the eye. How else are our eyes protected from any sudden light? By means of our eyelids, which may be closed, and thus shut out all the light. When we pass from a well-lighted room into the open air, at night, why does it seem darker at first than it does afterwards? Because the pupil is contracted at first; but it soon dilates, and allows more rays of light to pass into the eye, so that the night seems less dark.