In another form of bifocal lens, the surface of a single lens is ground to two different curvatures, by using a rotating hollow brass tube and emery to grind with, and an ebonite tube with rouge to polish with. If well done this lens is very satisfactory.
In another form, a hollow is first ground and polished in the crown lens; then a flint lens is ground and polished to the same curve. The flint is laid in the hollow in the crown lens, great care being taken to avoid including dust or air, and the two are fused together in an electric furnace. The outer surface is now ground all over to the same curvature, and the included flint lens is therefore almost invisible, but as it has a higher refractive index than the crown, this part of the lens will have a higher power, determined by the curvature of the dividing surface between the crown and the flint. This combination, being a convex flint and a concave crown, is non-achromatic. So lastly instead of a flint glass, a boro-silicate crown has been substituted, and as this glass has a lower dispersive power than the crown, although it has a higher refractive index, the combination is partly achromatized.
The refractive errors of the eyes to be corrected by spectacles are of three kinds: (i.) those due to malformation of the eye, (ii.) those due to old age, (iii.) those due to want of proper co ordination of the muscles of the eye.
(i.) The first of these may be again divided into two main types : (a) The curvature of the cornea or of the lens may be too great or too small in relation to the distance of the retina from them. This error can be corrected by the use of a concave or a convex spherical lens, respectively.
(b) The cornea, or sometimes the lens, may not be symmetri cal about its axis, being more curved in one meridian than in the one at right angles. This makes it impossible for the eye to form a point image of a bright point, the eye is therefore said to be "astigmatic." As already mentioned, this defect can be corrected by adding a cylindrical lens to increase the power of the weaker meridian, or to reduce that of the stronger, as may be more convenient. If the error was a large one, the eye will not have attempted to correct it ; but if it was small the eye will usually have tried to overcome it, and this will generally have caused more or less severe headaches. The prescription of the requisite glasses will remove these headaches.
In a number of cases the malformation of the one eye is different from that of the other. A child that has one good and
one bad eye, often learns to squint in order to throw the image in the bad eye out of the field. Correcting the bad eye in such a case at an early date will prevent the development of the squint.
In other cases both eyes may need correction, and it is usually possible to correct both eyes fully, if the case is taken early enough.
(ii.) The effect of age is gradually to lessen the ability of the lens of the eye to alter its power, and to see clearly objects at different distances. This loss of power continues progressively through life. In the case of a person with normal sight, at about the age of fifty he begins to find it impossible to focus anything nearer than about 25 in., and therefore reading begins to be diffi cult. A long-sighted person will find difficulty in reading at an earlier age. As a man gets older the addition required for reading will gradually become greater, and ultimately at about the age of 7o all power of altering the focus of the eye is lost. A short sighted person may never require glasses for reading.
(iii.) If some of the eye muscles are unequal so that the axes are normally not parallel to one another, the ganglia which co ordinate the movements of the eyes, will have a strain thrown upon them, which may often cause trouble. This is especially the case if the axis of one eye points higher up than the other, as we have not learnt to move the eyes up and down independently. In this case severe headache may result which a prism, or a decen tration of the lens, will relieve.
Lastly, spectacles may be required to cut off harmful rays of light. In the high Alps and the tropics, there is an excess of ultra-violet light, which has a harmful effect on the retina, and may even cause blindness; certain special glasses are on the mar ket which cut off these rays. Also in some occupations the workers are exposed to similar rays, as for instance in acetylene or arc-welding, and must protect their eyes either by wearing coloured glass goggles or by holding a screen containing coloured glass before their race. These goggles are generally glazed with ordinary blue or green glass, or sometimes with a coloured cellu loid film mounted between two plain glasses. Glass blowers and metal founders are exposed to great heat radiation, this is largely cut off by any glass, but to give a pleasanter light, a blue or green tinted glass is sometimes used. (R. S. CO