These powers of accommodation are however limited, and the sight is said to be perfect, when the eye can adapt itself to any distance within the usual limits, and when it cannot, vision is indistinct.
Defective sight arises from an incapa city of altering the position of the crys taline within the usual limits. 1. When it cannot be brought close enough to the cornea, near objects appear indistinct ; to this defect people in years are generally subject. 2. Where the crystaline cannot be drawn sufficiently near to the retina, remote objects appear indistinct ; this is the defect under which myopes, or short sighted people, labour. In each of these cases, the images of the different points in the object would be diffused over small circles on the retina ; and so being inter mixed and confounded with each other, would there form a very confused picture of the object : for in the former case (fig. 19), the image of any point would be formed behind the retina, as the re fraction of the eye is not sufficiently strong -to bring the rays (diverging so much as they do in proceeding from a near point) to a focus at the retina. This -defect will therefore be remedied by a convex glass, a b, which makes the point whence the rays now proceed more dis tant than the object ; therefore the rays falling on the eye will now diverge less than before, or else be parallel, and will of course be brought to a nearer focus, vi:. at the retina.
In the latter ease the image is formed before the retina (fig. 20) because the re fractive power of the eye is too great to permit rays so little diverging (as they do in proceeding from a distant point) to reach the retina before they are collect ed into a focus ; in this case the defect is supplied by a concave glass, a b, which makes the point whence the rays di verge, nearer than the object ; conse quently, the rays falling on the eye will now diverge more than before, so as when refracted through the humours not to come to their focus before they reach the retina.
Spectacles are constructed on the above principles, concave for short-sighted, and convex for long-sighted people. See SPEC TACLES and VISION.
"Of microscopes and other optical in struments." The impediments to the vi sion of very near objects arise from too great a divergence of the rays in each pencil incident on the eye, and are reme died by the microscope. This instru
ment is of two kinds : 1. refracting; and, 2. reflecting.
The refracting microscope is either single or compound. The former is a small double convex lens, of a short focal length ; the object is placed in its fo cus, by which disposition the rays of each pencil emerging from the lens be Caine parallel, and so are brought to their respective foci on the retina by the humours of the eye : the magnify ing power of the instrument appears from hence.
The apparent lineal magnitude of an object seen with this instrument, is to its lineal magnitude seen with the naked eye, as the least distance that admits of distinct vision with the naked eye, to the focal length of the lens ; for these magni tudes are as the angles under which the object appears, i. e. inversely as the dis tances at which it is viewed.
A compound microscope -is composed of two double convex glasses, the broader next the eye. In this instrument the dis tance of the object from the object-glass is to be made greater than the focal length of that lens ; then the image will be formed at the focus conjugate to the place of the object, and the eye-glass being placed at its own focal distance from the image, will make the rays emerge parallel to each other, and consequent ly produce distinct vision. See Mica°. SCOPE.
To enlarge the field of the compound microscope, it is usual to insert a broad lens, as in the astronomical telescope, be tween the object-glass and the image.
The reflecting microscope is thus con structed : In the extremity of a broad tube insert a concave speculum N U (fig. 21) ; a point 0 in its axis, whose distance from the vertex, V, is greater than the focal length of the concave, is the place for the object, whose image will conse quently be formed at the focus, G, con jugate to the point 0 : at the distance of its own focal length, L G, place a double convex lens, a b, by which the image will be seen distinctly. The object is illumi nated by light admitted into the tube through a space, P It, adjoining to the speculum ; and the illustration of the ob ject may be rendered more intense by a concave speculum, A B, which shall re flect the light so admitted to a focus at the place of the object.