Optical glass

“ Since the employment of glass in optics depends upon its ability to bend the rays of light to a common point or focus, its value increases with its refractive power—that is to say, with its density. The problem set before the maker of optical glass is, therefore, quite different from that which must be solved by the manufacturer of more every-day goods. He must produce a glass which has great weight without any loss of transparency. The difficulty lies in this, that the substances which add weight to the glass are prejudicial to its transparency. ”

“ While the more ordinary kinds of glass, having average optical properties and showing dispersive powers roughly conforming to the law of inverse proportionality with refractive index which governs the older varieties of optical glass, are chemically stable substances, showing little tendency to undergo either chemical changes or to crystallise during cooling, the more extreme glasses exhibit these undesirable features to an increasing extent the more nearly the limit of our present range is approached. ”

“ Added to all these trials of the lens maker is the one outstanding fact that the process can never be hurried at any stage, the efficient annealing of optical glass is one of the most important stages in its manufacture. ”

“ Again, glass is cheap, being literally made from the dust of the earth; it is transparent, and so can be used in buildings, transmitting light whilst protecting from the inclemency of the weather. Its transparency, too, combined with its high refractive power,{11} make it of inestimable value in the manufacture of optical instruments. It is this high refractive power, too, which gives to cut glass its beautiful lustre and sparkle, and one aim of the glass-founder is to increase this refractive power and so enhance the brilliancy of his product. ”

“ It will be readily understood from the above account of the process of production that optical glass, relatively to other kinds of glass, is very expensive, the actual price varying from 3s. to 30s. per ℔ in small slabs or disks. The price, however, rapidly increases with the total bulk of perfect glass required in one piece, so that large disks of glass suitable for telescope objectives of wide aperture, or blocks for large prisms, become exceedingly costly. ”

“ The fundamental difference between the making of optical glass and the ordinary commercial varieties lies in the individual treatment of each charge necessary to secure uniformity and regularity, carried even to the extent of cooling each melting very slowly in its own pot, which is finally broken up to recover the contents. ”

“ A consideration of the various factors that are involved in the production of a piece of perfect optical glass will make it apparent that the cost and difficulty of its production237 increases rapidly with the weight of the piece to be produced, so that it is not surprising to find that the price of very large discs of perfect optical glass such as those required for large astronomical telescopes, reaches figures which become prohibitive when very large sizes are considered. ”

“ M. Faraday.
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The optical glass was a failure, so far as concerned the original hope that it would lead to great improvements in telescopes. Nevertheless it furnished scientific men with a new material, the “heavy glass” consisting essentially of boro-silicate of lead, for which sundry uses in spectroscopy and other optical instruments have since been found. ”

“ The cover slip is not only a protection for our object but for our objective. Water you may argue cannot harm the lenses of the objective. Perhaps not, we will not argue the point but, when the water dries on the objective, it leaves a certain amount of deposit on the glass and this deposit must be rubbed off. The less often the lenses are rubbed the better for them, glass especially very highly polished optical glass, is far more easily scratched than many [299] people imagine and a scratched lens is an inefficient lens. ”

“ Some combinations are more likely to give trouble from this cause than others, and while [Pg 58] a very wide variety of oxides can be coerced into solution with silica, a comparatively limited number produce a homogeneous and colorless glass useful for optical purposes.
Many mixtures entirely suitable for common commercial purposes are out of the question for lens making, through tendency to surface deterioration by weathering, lack of homogeneous quality, or objectionable coloration. ”

“ It is to be hoped, however, that makers of optical glass will not relax their efforts till astronomers shall be able to obtain refracting telescopes in which the secondary spectrum is nearly if not quite eliminated. ”

“ From the large masses great lenses and mirrors may be produced, while the smaller pieces are used for the production of the disks and slabs of moderate size, in which the optical glass of commerce is usually supplied. In order to allow of the removal of the glass, the cold crucible is broken up and the glass carefully separated from the fragments of fireclay. ”

“ From this brief account of the manufacture of optical glass it is clear that a good lens is always worth a high price. Its components must be of the purest quality obtainable, the process of manufacture requires highly skilled labour and it is laborious and exacting work. Constant attention is needed from start to finish and much of the glass is never sufficiently good to pass the rigorous tests which it must undergo. ”

“ Glass manufacturers seem to experience unusual difficulties in their attempts to form large discs of optical glass pure enough and uniform enough to be suitable for telescopes. These difficulties are enhanced with every increase in the size of the discs, so that the cost has a tendency to increase at a very much greater rate. It may be mentioned in illustration that the price paid for the object-glass of the Lick telescope exceeded ten thousand pounds. ”

“ Chromatic aberration is easily cured by using an achromatic or compound lens made by cementing together two lenses of crown and flint glass respectively, as in Fig. 40.
It will be seen that the flint glass component by itself is a concave lens and therefore neutralises in part, or in whole, the convex crown lens. Flint glass has both greater dispersive power and also greater refractive power than crown glass, but fortunately not to the same degree ”

“ Optical glass, which may be made by softening down irregular fragments into moulds at a temperature below that of fusion, may have the same difficulty, but whether it has a diameter of minimum compression can only be determined by experiment. Why speculum metal should have the same property might be ascertained by a critical examination of the process of casting, and the effect of the position of the openings in the mould for the entrance of the molten metal. ”

“ Years ago, there was a little instrument of a similar kind, sold for a penny in the streets of London, which consisted of a morsel of looking-glass set at an angle, in a pill-box, and which gave the possessor the power of seeing all that was going on behind him. Persons who wear dark preservers are often in the habit of observing all that is going on behind their backs by the reflection seen in the corner of their glasses.
Such are the principal optical recreations founded on the reflecting and refracting properties of mirrors and lenses. ”

“ Now the optical law which most affects the technique of stained glass is that of which the effect is known as "irradiation." In an unscientific work it is enough to say that it is the law which causes the filament of an electric light, in reality thin as a hair, to appear when incandescent as thick as a piece of worsted. In the same way it makes the clear spaces of glass appear larger than they really are in proportion to the obscured parts, and also tends to make them look rounded. ”

“ An equal gain has followed upon varying anew the composition of glass. For centuries the only materials added to sand for its melting pot were silicic acid, potash, soda, lead-oxide, and lime. As optical research grew more exacting the question arose, Will new ingredients give us lenses of better qualities? First of all came the demand for glasses which combined in lenses would yield images in the telescope and microscope free from color. In a simple lens, such as that of an ordinary reading glass, we can readily observe the production of color by a common beam of light. ”

“ The Lens also ought to be good, such as may serve for optical Uses, and the Prism ought to have a large Angle, suppose of 65 or 70 Degrees, and to be well wrought, being made of Glass free from Bubbles and Veins, with its Sides not a little convex or concave, as usually happens, but truly plane, and its Polish elaborate, as in working Optick-glasses, and not such as is usually wrought with Putty, whereby the edges of the Sand-holes being worn away, there are left all over the Glass a numberless Company of very little convex polite Risings like Waves. ”

“ Since homogeneity is so essential in the finished product, very thorough mixing of the raw materials is necessary in the case of optical glass, and the ingredients are for this purpose generally used in a state of finer division than is necessary with other varieties of glass. As a rule the quantities of mixture of any one kind that are required are not large enough to justify the use of mechanical appliances, and very careful hand-mixing is carried out. ”

“ The existence of internal strains in glass can be readily recognized by examination in polarized light, any signs of double refraction indicating the existence of strain. If the glass is very badly annealed, the lenses made from it may fly to pieces during or after manufacture, but apart from such extreme cases the optical effects of internal strain are not readily observed except in large optical apparatus. ”

“ Variations in thickness of this film of silver on various parts of the face of the mirror are consequently only small fractions of 1/200,000 of an inch, and are therefore of no optical moment whatever. If a glass has been properly silvered, and shows the sun of the same color and intensity through all parts of its surface, the most delicate optical tests will certainly fail to indicate any difference in figure between the silver and the glass underneath. ”

“ The raw materials are selected with great care to assure chemical purity, but whereas in most glasses the only impurities to be dreaded are those that are either infusible or produce a colouring effect upon the glass, for optical purposes the admixture of other glass-forming bodies than those which are intended to be present must be avoided on account of their effect in modifying the optical constants of the glass. ”

“ The study of light and vision, and the need for better methods and instruments for observing life resulted in time in the invention of the first optical device—the magnifying glass. All telescopes, microscopes, spectacles, cameras, projectors and other optical instruments have been evolved from the simple lens or magnifying glass. That lens was a special boon to the men and women who through birth, age or misfortune had poor eyesight. ”

“ The best field glasses have each objective a triple cemented lens, and the concave lenses also [Pg 151] triplets, the arrangement of parts being that shown in Fig. 110. Glasses of this sort rarely have a magnifying power above 5.
In selecting a field glass with the idea of using it on the sky try it on a bright star, real or artificial, and if the image with careful focussing does not pull down to a pretty small and uniform point take no further interest in the instrument.
Fig. 110.—Optical Parts of Field Glass.
The advantage of a binocular instrument is popularly much exaggerated. ”

“ Through it at the height of the brightest part of the flame, as at b, two holes are bored, a quarter of an inch apart, one 1/32 of an inch in diameter, the other as small as the point of the finest needle will make—perhaps 1/200 of an inch. This apparatus is to be set at the centre of curvature of the mirror c—the optical axis of the latter being horizontal—and so adjusted that the light which diverges from the illuminated hole in use, may, after impinging on the concave surface of the glass, return to form an image close by the side of the tin screen. ”

“ Such accidents, too, teach a useful lesson in the management of a large telescope, never, for instance, to leave one-half the mirror or lens exposed to radiate into cold space, while the other half is covered by a comparatively warm dome. Under the head of the Sun-Camera, some further facts of this kind may be found.
Oblique Mirrors.—Still another propensity of glass and speculum metal must be noted. A truly spherical concave can only give an image free from distortion when it is so set that its optical axis points to the object and returns the image directly back towards it. ”

“ In another direction the employment of quartz lenses makes it possible to take advantage of the optical properties of ultra-violet light in connection with microscopy; for the purpose of constructing a perfect optical system, crystalline quartz244 would be useless, since its property of double refraction would interfere hopelessly with the performance of the lenses. This is now overcome by the use of vitreous silica lenses, in the case of the “ultra-violet microscope,” as made by Carl Zeiss, of Jena. ”

“ Were it attempted to manufacture the large glass bodies required for lighthouse purposes in this manner, the cost would approximate to that of the large discs used251 for telescope objectives, and this would of course be entirely prohibitive. The requirements as regards colour, homogeneity and freedom from other defects, which must be met in lighthouse lenses, are further not nearly so stringent as those which are essential in ordinary optical work of good quality. ”