It is clear that the chance of getting a large and perfect chunk from the crucible is far smaller than that of getting fragments of a few pounds, so that the production of a perfect disc for a large objective requires both skill and luck. Little wonder therefore that the price of discs for the manufacture of objectives increases substantially as the cube of the diameter.
The process of optical glass making as here described is the customary one, used little changed since the days of Guinand. The great advances of the last quarter century have been in the production of new varieties having certain desirable qualities, and in a better understanding of the conditions that bring a uniform product of high quality. During the world war the greatly increased demand brought most extraordinary activity in the manufacture, and especially in the scientific study of the problems involved, both here and abroad. The result has been a long step toward quantity production, the discovery that modifications of the tank process could serve to produce certain varieties of optical glass of at least fair quality, and great improvements in the precision and rapidity of annealing.
These last are due to the use of the electric furnace, the study of the strains during annealing under polarized light, and scientific pyrometry. It is found that cooling can be much hastened over certain ranges of temperature, and the total time required very greatly shortened. It has also been discovered, thanks to captured instruments, that some of the glasses commonly regarded as almost impossible to free from bubbles have in fact yielded to improved methods of treatment.
Conventionally optical glass is of two classes, crown and flint. Originally the former was a simple compound of silica with soda and potash, sometimes also lime or magnesia, while the latter was rich in lead oxide and with less of alkali. The crown had a low index of refraction and small dispersion, the flint a high index and strong dispersion. Crown glass was the material of general use, while the flint glass was the variety used in cut glass manufacture by reason of its brilliancy due to the qualities just noted.
Fig. 38.—The Index of Refraction.
The refractive index is the ratio between the sine of the angle of incidence on a lens surface and that of the angle of refraction in passing the surface. Fig. 38 shows the relation of the incident and refracted rays in passing from air into the glass lens surface L, and the sines of the angles which determine n, the conventional symbol for the index of refraction. Here i is the angle of incidence and r the angle of refraction i.e. n = s/s′. The indices of refraction are usually given for specific colors representing certain lines in the spectrum, commonly A¹, the potassium line in the extreme red, C the red line due to hydrogen, D the sodium line, F the blue hydrogen line and G′ the blue-violet line hydrogen line, and are distinguished as nc, nd, nf, etc. The standard dispersion (dn) for visual rays is given as between C and F, while the standard refractivity is taken for D, in the bright yellow part of the spectrum. (Note. For the convenience of those who are rusty on their trigonometry, Fig. 39 shows the simpler trigonometric functions of an angle. Thus the sine of the angle A is, numerically, the length of the radius divided into the length of the line dropped from the end of the radius to the horizontal base line, i.e. bc/Ob, the tangent is da/Ob, and the cosine Oc/Ob.)
Ordinarily the index of refraction of the crown was taken as about 3/2, that of the flint as about 8/5. As time has gone on and especially since the new glasses from the Jena works were introduced about 35 years ago, one cannot define crowns and flints in any such simple fashion, for there are crowns of high index and flints of low dispersion.
Fig.39.—The Simple Trigonometric Functions of an Angle.