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ON THE FORMATION OF LENSES.
PREPARATION OF THE NECESSARY TOOLS. CHOICE OF GLASS. GRINDING. — POLISHING, -CÚRDLED LENSES. - MEANS USED FOR AVOIDING THIS DEFECT.
In grinding glasses for spectacles, or preparing them as lenses for optical instruments, the first thing to be attended to is to determine the proper focal distance of the glass. Taking then a pair of compasses, which, on the supposition that the glass is intended to be convex or concave on both sides, must be opened to the full focal distance; two arches or segments of circles, each extended somewhat beyond the breadth which it is intended to give to the glass, must be described upon a piece of sheet copper, which must then be filed away from the outside of one and from the inside of the other arch. By this means two gauges are formed, the one convex and the other concave, and each perfectly answering to the other.
If it is intended that the glass shall be what the opticians call plano-convex or plano-concave, that is, having one of its sides flat, while the other has received the requisite curvature, the compasses wherewith the arches are described should be opened to only one half the focal distance.
Two circular plates of brass, about one tenth of an inch in thickness, and each being of a little larger diameter than the intended lenses, are then securely soldered upon a cylindrical piece of lead of an equal diameter with the brass discs, and one inch in thickness; these, which are called tools, are then fixed in the lathe, and turned so as to correspond with the copper gauges, the surface of one being made convex, and of the other concave.
The two brass discs are then to be ground together with emery, or with pounded Turkey stone, until their surfaces exactly coincide in every point.
If the focal distance is very short, so that the convexity and concavity require to be very considerable, the brass discs should be hammered as nearly as possible to their intended form before they are soldered to the leaden cylinders, and turned, otherwise either the thickness of the brasses would require to be inconveniently increased, or the more considerable portion of their substance, which must in such case be cut away, would occasion the discs to be too thin and yielding.
The glass of which a lens is composed is chosen with reference to the purpose to which it is to be applied, and according to its refractive and dispersive powers: its selection must be left to the discretion of the optician. Its two surfaces should originally be perfectly parallel. Being cut or clipped into a circular form by means of scissors or pincers, the edge must be smoothed on a common grindstone, and the glass fixed by seating one of its surfaces in softened pitch on the flat end of a solid, cylindrical, wooden handle of smaller diameter than the glass. The centre of the axis of this handle must coincide exactly with the centre of the glass.
If, to suit a short focal distance, the curvature of the lens requires to be great, it will simplify the labour of the artist, if, previously to its being thus fitted to its handle, the glass is reduced upon the grindstone as nearly as possible to the shape of the gauge. Some judgment is, however, necessary in this process, lest the abrasion should be carried too far even in any one minute point, which would render the glass wholly unserviceable.
The convex form is that which is most commonly given to lenses; and in describing the process for effecting this, the mode of producing concave glasses will equally be understood ; the only difference between the two methods being this,—that in the first operation, the concave tool and gauge are brought into use; while for the other, those having a convex form are employed. The whole being thus prepared, the concave tool is fixed firmly on the working bench; and having some fine emery sprinkled on its surface, the glass is worked upon it with circular and cross strokes alternately; the artist being careful that the centre of the glass shall never pass beyond the edge of the tool.
When by these means the glass has been so far ground that its surface coincides with that of the tool at every point, the emery is to be washed away, and some of a finer kind substituted; and so on through three or four different degrees of fineness, until all the roughnesses and apparent scratches on the glass are worn down, and it has become perfectly smooth to the touch, although dull and opaque to the eye: after this it is sometimes further ground with finely pounded pumice-stone.
At the expiration of every five or six minutes, during this grinding process, the surface of the tool is rubbed for a short time within the concave tool, that its proper curvature may be perfectly preserved. When the operation has been completed, the glass is easily separated from its wooden handle by means of a thin knife, and the pitch is removed by rubbing it with oil. The side which has been ground is, in its turn, fixed upon the wooden handle, and the other side is then ground in the same manner as the first.
Convex glasses are frequently prepared for common purposes, in another manner. The concave tool is fixed upon the lathe, and the glass being held steadily in the hand, and sprinkled with emery, is applied to the tool during its revolutions. For concave glasses, the convex tool is fitted to the lathe, and the glass is in like manner presented to it; but this method, although easier and more expeditious, is greatly inferior in its result to hand-grinding, and cannot be resorted to when any thing like perfectness in the intended instrument is desired.
The same brass tool which is used for grinding, serves also for polishing lenses ; but before it is thus employed, a smooth thick piece of felt must be stretched over and cemented to it, and the outer surface being then covered
with washed putty powder, which is a combination of the oxides of tin and lead, the tool is worked upon the lens with the same motions as are employed in grinding it. The consistency of the powder is a point requiring attention ; for if it be too moist, it will cause the fibres of the felt to rise up and polish, not only the surface, properly speaking, but likewise the innumerable hollows, which, notwithstanding all appearances to the contrary, are actually left in the surface from the grinding. If the lens be subjected to examination in a microscope, this effect will be rendered fully apparent. The evil consequence resulting from this defect is, that the cavities being polished, admit the rays of light, and disperse, instead of collecting them, as would be the case if the surface were uniform. When this fault exists in a degree so exaggerated as to be visible to the naked eye, the lens is said to be curdled.
An excellent method has lately been adopted by an eminent optician in London, whereby this defect is avoided. Bees' wax is hardened to a proper degree by admixture with dry red sulphate of iron, which has previously been carefully washed ; and instead of the covering of felt, this compound is melted over the brass tool. When cold, the casing thus formed is sufficiently hardened to be turned to the required curvature, and the tool, when this has been done, is in a fit state for use.
The peculiar advantage of this compound, as a polishing substance, consists in its perfect uniformity ; besides which, it has this further recommendation, that if any hard particles should accidentally insinuate themselves between the tool and the lens, and which, in other cire cumstances would scratch the glass, the wax is suffis ciently yielding to allow them to bury themselves in its substance, so that all injury of this kind is avoided.
Lenses which have been thus treated, will bear ex'amination with a microscope, their polish appearing uniformly clear and defined.
Convex lenses in their simple state have been used
for collecting the heating rays of the sun, or for forming what are called burning glasses. One of the largest lenses ever applied to this purpose was made of fint glass by Mr. Parker. The diameter of this glass was 3 feet; its focal distance was 45 inches ; and the circular spot of light which it cast at the focal point was 1 inch in diameter. Still farther, and as much as possible to condense the rays, Mr. Parker employed a smaller lens, 13 inches in diameter, in conjunction with the larger one, and by means of this the heating rays were concentrated at the focal point to of an inch. The effects produced by this arrangement were surprising ; 20 grains of pure gold were fused in 4 seconds, the same effect was produced on 10 grains of platina in 3 seconds; and a diamond, whose weight was 10 grains, was found to have lost 4 grains after having been placed within the focus during 30 minutes.
This lens, which cost 7001., has since passed into the possession of the emperor of China.