Optical Properties of Glass.—The most essential property of glass in this respect is homogeneity. We have already indicated that glass can never be regarded as a definite chemical substance or compound, but that it usually consists of mutual solutions of various complex silicates, borates, etc. Solutions being of the very nature of mixtures of two or more different substances, it follows that they can only become homogeneous when complete mixing has taken place. We have a familiar example of the formation of such a solution when sugar is dissolved in water. The water near the sugar becomes saturated with sugar and of different density from the remaining water; if the liquid is slightly stirred a very characteristic phenomenon makes its appearance—the pure water and the dense sugar solution do not at once mix completely, the denser liquid remaining for a time disseminated throughout the whole fluid mass in the form of more or less fine lines, sheets, or eddies, and these are visible because the imperfectly mixed liquids have different effects on the light passing through them. In the case of sugar-water we are, however, dealing with a very mobile liquid, and a few turns of a tea-spoon suffice to render the mixture complete, and the liquid, which for a few moments had appeared turbid, becomes homogeneous and transparent. In the case of glass, when the raw materials are melted together, a mixture is formed of liquids of differing densities similar to that which was temporarily formed in the sugar-water solution. Molten glass, however, is never so mobile a liquid as ordinary water, nor is it in the ordinary course of manufacture subjected to any such thorough mixing action as that which is produced by a spoon in a glass of water. In glass as ordinarily manufactured, therefore, it is not surprising to find that the lack of homogeneity which originates during the melting persists to the end. Its effects can be traced whenever a thick piece of ordinary glass is carefully examined, when the threads or layers of differing densities can be recognised in the form of minute internal irregularities in the glass. These defects are known as striæ or veins, and their presence in glass intended for the better kind of optical work renders the glass useless. As will be seen below in the production of optical glass, special means are adopted for the purpose of rendering it as homogeneous as possible; in fact, the early history of optical glass manufacture is simply the history of attempts to overcome this very defect. The problem is, however, beset by chemical and physical difficulties of no mean order, and even in the best modern practice only a small proportion of each melting or crucible full of glass is entirely free from veins or striæ. In many cases these defects are very minute, and sometimes escape observation until the stage of the finished lens is reached. At that stage, however, their presence becomes painfully evident from the fact that they interfere seriously with the sharp definition of the images formed by the lens in question. It will be seen that in such a case time and money has been wasted by grinding and polishing what turns out to be a useless piece of glass. Methods are, therefore, used for examining the glass before it is worked, whereby the existence of the smallest striæ can scarcely escape detection. These methods depend upon the principle that a beam of parallel light passing through a plate of glass will meet with no disturbance so long as the glass is homogeneous, but if striæ are present, they will cause the light to deviate from parallelism wherever it falls upon them. Under such illumination, therefore, the striæ will appear as either dark or bright lines, when they can be readily detected. One form of apparatus used for this purpose is illustrated in [Fig. 14].
Fig. 14.—Diagram of striæ-testing apparatus.
L, source of light; S, slit; A and B, simple convex lenses; G, glass under test; E, eye of observer. The arrows indicate the paths of light-rays.
Transparency and colour are obviously fundamentally important properties of glass. In one sense homogeneity is essential to transparency, but the aspect of the subject which we are now considering is that of the absorption of light in the course of regular transmission through glass. It may be said at once that no glass is either perfectly transparent or, what comes to nearly the same thing, perfectly free from colour. In the case of the best optical glasses it is true that the absorption of light is very slight, but even these, when considerable thicknesses are viewed, show a greenish-yellow or bluish colouring. On the other hand, certain optical glasses which are used at the present time for many of our best lenses absorb light so strongly or are so deeply coloured that a thickness of a few inches is sufficient to reveal this defect. To some extent public taste or opinion which objects to the use of even a slightly greenish glass in optical instruments of good quality is to blame for the tint of these glasses. In many cases glass-makers could produce a very slightly greenish glass, but in order to overcome this colour they deliberately add to the glass a colouring oxide imparting to the glass a colour more or less complementary to the natural green tint. The result is a more or less neutral-tinted glass which, however, absorbs much more light than the naturally green glass would have done. Since such glass is frequently used for photographic lenses, it is interesting to note that the light rays whose transmission is sacrificed in order to avoid the green tint are those lying at or near the blue end of the spectrum, so that the photographic rapidity of the resulting lenses is decidedly reduced by the use of such glass.
Refraction and Dispersion.—The quantitative properties of glass, governing its effect upon incident and transmitted light, are, of course, of fundamental importance in all its optical uses. The fundamental optical constant of each variety of optical glass is known as its refractive index; this number really represents the ratio of the velocity with which light waves are propagated through the glass to the velocity with which they travel through free space. Not only does this ratio vary with every change in the chemical composition and physical condition of the glass, but it also varies according to the length of the light waves themselves. In other words, the short waves of blue light are transmitted through glass with a different velocity from that with which the longer waves of red light are transmitted. The consequence is that when a beam of white light is passed through a prism it is split up and spread out into a number of beams representing all the colours of the spectrum in their proper order, the blue light suffering the greatest deflection from its original path, while the red light suffers least deflection. Both the actual and relative amount by which light rays of various colours are deflected under such circumstances depends upon the nature of the glass in question; therefore, to fully characterise the optical properties of a given kind of glass it is necessary to state not only its refractive index but to specify the refractive indices for a sufficient number of different wave-lengths of light, suitably distributed through the spectrum. For this purpose a number of well-marked spectrum lines have been chosen, the systematic use of the particular set of lines which is now usually employed being due to the initiative of Abbé and Schott at Jena, who initiated the system of specifying the optical properties of glass in this way. The actual lines chosen are the line known as A′, corresponding to a wave-length of 0·7677 micro-millimetres, and the lines known as C, D, F, and G′, whose wave-lengths, in the same units, are 0·6563, 0·5893, 0·4862, and 0·4341 respectively. The A′ line, however, lies so near the extreme red end of the spectrum that the data concerning it are seldom required.
As a matter of fact, the actual refractive index is only stated in most tables of optical glasses for sodium light (D line), the dispersive properties of the glass being indicated by tabulating the differences between the refractive indices for the various lines, the table thus containing columns marked C-D, D-F, F-G′. These figures are usually described as the “dispersion” of the glass from C to D, D to F, etc. In addition to these figures it is usual to tabulate what is called the “mean dispersion” of the glass, which is simply the difference between the refractive indices for C and F lines; this interval is usually taken as representing that part of the spectrum which is of the greatest importance for visual purposes. A further constant which is of great importance in the calculations for achromatic lenses is obtained by dividing the mean dispersion into the refractive index for the D line minus one (usually written (C-F)/(nD-1)=ν). This term, for which no satisfactory name has yet been suggested, characterises the ratio of the dispersive power of the glass to its total refracting power. It is usually denoted by the Greek letter ν. The following table (taken from the Catalogue of the Optical Convention, 1905) gives a list of optical glasses produced by Messrs. Chance, of Birmingham. This list, although it is not nearly so long as that issued by the French and German firms who manufacture optical glass, contains examples of the most important types of optical glass which are available at the present time. Those, however, who wish to use the data for the purpose of lens calculation are advised to consult the latest issues of the optical glass-makers’ catalogues, since the range of types available, and even the actual figures for some of the glasses, are liable to variation from time to time.
Table of Optical Properties.
| Name. | nD. | ν. | Medium Dispersion. C–F. | Partial and Relative Partial Dispersions. | |||||
|---|---|---|---|---|---|---|---|---|---|
| C–D. | C–D —— C–F. | D–F. | D–F —— C-F. | F–G′. | F–G′ —— C–F. | ||||
| Extra Hard Crown | 1·4959 | 64·4 | ·00770 | ·00228 | ·296 | ·00542 | ·704 | ·00431 | ·560 |
| Boro-silicate Crown | 1·5096 | 63·3 | ·00803 | ·00236 | ·294 | ·00562 | ·700 | ·00446 | ·555 |
| Hard Crown | 1·5175 | 60·5 | ·00856 | ·00252 | ·294 | ·00604 | ·706 | ·00484 | ·554 |
| *Medium Barium Crown | 1·5738 | 57·9 | ·00990 | ·00293 | ·296 | ·00697 | ·704 | ·00552 | ·557 |
| *Densest Barium Crown | 1·6065 | 57·9 | ·01046 | ·00308 | ·294 | ·00738 | ·705 | ·00589 | ·563 |
| Soft Crown | 1·5152 | 56·9 | ·00906 | ·00264 | ·291 | ·00642 | ·708 | ·00517 | ·570 |
| *Medium Barium Crown | 1·5660 | 56·3 | ·01006 | ·00297 | ·295 | ·00709 | ·704 | ·00576 | ·572 |
| Barium Light Flint | 1·5452 | 53·5 | ·01020 | ·00298 | ·292 | ·00722 | ·701 | ·00582 | ·570 |
| Extra Light Flint | 1·5316 | 49·0 | ·01085 | ·00313 | ·288 | ·00772 | ·711 | ·00630 | ·580 |
| Extra Light Flint | 1·5333 | 48·5 | ·01099 | ·00322 | ·293 | ·00777 | ·707 | ·00640 | ·582 |
| Boro-silicate Flint | 1·5623 | 47·4 | ·01187 | ·00343 | ·289 | ·00844 | ·711 | ·00693 | ·584 |
| *Barium Light Flint | 1·5833 | 46·6 | ·01251 | ·00362 | ·288 | ·00889 | ·711 | ·00721 | ·576 |
| Soda Flint | 1·5482 | 45·8 | ·01195 | ·00343 | ·287 | ·00852 | ·713 | ·00690 | ·577 |
| Light Flint | 1·5472 | 45·8 | ·01196 | ·00348 | ·291 | ·00848 | ·709 | ·00707 | ·591 |
| Light Flint | 1·5610 | 43·2 | ·01299 | ·00372 | ·287 | ·00927 | ·713 | ·00770 | ·593 |
| Light Flint | 1·5760 | 41·0 | ·01404 | ·00402 | ·286 | ·01002 | ·713 | ·00840 | ·598 |
| Light Flint | 1·5787 | 40·7 | ·01420 | ·00404 | ·284 | ·01016 | ·715 | ·00840 | ·591 |
| Dense Flint | 1·6118 | 36·9 | ·01657 | ·00470 | ·284 | ·01187 | ·716 | ·01004 | ·606 |
| Dense Flint | 1·6214 | 36·1 | ·01722 | ·00491 | ·285 | ·01231 | ·715 | ·01046 | ·608 |
| Dense Flint | 1·6225 | 36·0 | ·01729 | ·00493 | ·286 | ·01236 | ·715 | ·01054 | ·609 |
| Extra Dense Flint | 1·6469 | 33·7 | ·01917 | ·00541 | ·285 | ·01376 | ·720 | ·01170 | ·655 |
| Densest Flint | 1·7129 | 29·9 | ·02384 | ·00670 | ·281 | ·01714 | ·789 | ·01661 | ·678 |