It is a further striking fact, not unconnected with those just enumerated, that the extreme range of optical properties covered even by the relatively large number of optical glasses now available is in reality very small. The refractive indices of all glasses at present available lie between 1.46 and 1.90, whereas transparent minerals are known having refractive indices lying considerably outside these limits; at least one of these, fluorite (calcium fluoride), is actually used by opticians in the construction of certain lenses, so that probably progress is to be looked for in a considerable widening of the limits of available optical materials; possibly such progress may lie in the direction of the artificial production of large mineral crystals.

The qualities required in optical glasses have already been partly referred to, but may now be summarized:—

1. Transparency and Freedom from Colour.—These qualities can be readily judged by inspection of the glass in pieces of considerable thickness, and they may be quantitatively measured by means of the spectro-photometer.

2. Homogeneity.—The optical desideratum is uniformity of refractive index and dispersive power throughout the mass of the glass. This is probably never completely attained, variations in the sixth significant figure of the refractive index being observed in different parts of single large blocks of the most perfect glass. While such minute and gradual variations are harmless for most optical purposes, sudden variations which generally take the form of striae or veins are fatal defects in all optical glass. In their coarsest forms such striae are readily visible to the unaided eye, but finer ones escape detection unless special means are taken for rendering them visible; such special means conveniently take the form of an apparatus for examining the glass in a beam of parallel light, when the striae scatter the light and appear as either dark or bright lines according to the position of the eye. Plate glass of the usual quality, which appears to be perfectly homogeneous when looked at in the ordinary way, is seen to be a mass of fine striae, when a considerable thickness is examined in parallel light. Plate glass is, nevertheless, considerably used for the cheaper forms of lenses, where the scattering of the light and loss of definition arising from these fine striae is not readily recognized.

Bubbles and enclosures of opaque matter, although more readily observed, do not constitute such serious defects; their presence in a lens, to a moderate extent, does not interfere with its performance (see above).

3. Hardness and Chemical Stability.—These properties contribute to the durability of lenses, and are specially desirable in the outer members of lens combinations which are likely to be subjected to frequent handling or are exposed to the weather. As a general rule, to which, however, there are important exceptions, both these qualities are found to a greater degree, the lower the refractive index of the glass. The chemical stability, i.e. the power of resisting the disintegrating effects of atmospheric moisture and carbonic acid, depends largely upon the quantity of alkalis contained in the glass and their proportion to the lead, lime or barium present, the stability being generally less the higher the proportion of alkali. A high silica-content tends towards both hardness and chemical stability, and this can be further increased by the addition of small proportions of boric acid; in larger quantities, however, the latter constituent produces the opposite effect.

4. Absence of Internal Strain.—Internal strain in glass arises from the unequal contraction of the outer and inner portions of masses of glass during cooling. Processes of annealing, or very gradual cooling, are intended to relieve these strains, but such processes are only completely effective when the cooling, particularly through those ranges of temperature where the glass is just losing the last traces of plasticity, is extremely gradual, a rate measured in hours per degree Centigrade being required. 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. Very perfectly annealed optical glass is now, however, readily obtainable.

5. Refraction and Dispersion.—The purely optical properties of refraction and dispersion, although of the greatest importance, cannot be dealt with in any detail here; for an account of the optical properties required in glasses for various forms of lenses see the articles [Lens] and [Aberration]: II. In Optical Systems. As typical of the range of modern optical glasses Table I. is given, which constituted the list of optical glasses exhibited by Messrs Chance at the Optical Convention in London in 1905. In this table n is the refractive index of the glass for sodium light (the D line of the solar spectrum), while the letters C, F and G′ refer to lines in the hydrogen spectrum by which dispersion is now generally specified. The symbol ν represents the inverse of the dispersive power, its value being (nD − 1)/(C − F). The very much longer lists of German and French firms contain only a few types not represented in this table.

Table I.—Optical Properties.

Factory
Number.		 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.
C.   644 Extra Hard Crown 1.4959 64.4 .00770 .00228 .296 .00542 .704 .00431 .560
B.   646 Boro-silicate Crown 1.5096 63.3 .00803 .00236 .294 .00562 .700 .00446 .555
A.   605 Hard Crown 1.5175 60.5 .00856 .00252 .294 .00604 .706 .00484 .554
C.   577 Medium Barium Crown 1.5738 57.9 .00990 .00293 .296 .00697 .704 .00552 .557
C.   579 Densest Barium Crown 1.6065 57.9 .01046 .00308 .294 .00738 .705 .00589 .563
A.   569 Soft Crown. 1.5152 56.9 .00906 .00264 .291 .00642 .708 .00517 .570
B.   563 Medium Barium Crown 1.5660 56.3 .01006 .00297 .295 .00709 .704 .00576 .572
B.   535 Barium Light Flint 1.5452 53.5 .01020 .00298 .292 .00722 .701 .00582 .570
A.   490 Extra Light Flint 1.5316 49.0 .01085 .00313 .288 .00772 .711 .00630 .580
A.   485 Extra Light Flint 1.5333 48.5 .01099 .00322 .293 .00777 .707 .00643 .582
C.   474 Boro-silicate Flint 1.5623 47.4 .01187 .00343 .289 .00844 .711 .00693 .584
B.   466 Barium Light Flint 1.5833 46.6 .01251 .00362 .288 .00889 .711 .00721 .576
B.   458 Soda Flint 1.5482 45.8 .01195 .00343 .287 .00852 .713 .00690 .577
A.   458 Light Flint 1.5472 45.8 .01196 .00348 .291 .00848 .709 .00707 .591
A.   432 Light Flint 1.5610 43.2 .01299 .00372 .287 .00927 .713 .00770 .593
A.   410 Light Flint 1.5760 41.0 .01404 .00402 .286 .01002 .713 .00840 .598
B.   407 Light Flint 1.5787 40.7 .01420 .00404 .284 .01016 .715 .00840 .591
A.   370 Dense Flint 1.6118 36.9 .01657 .00470 .284 .01187 .716 .01004 .606
A.   361 Dense Flint 1.6214 36.1 .01722 .00491 .285 .01231 .715 .01046 .608
A.   360 Dense Flint 1.6225 36.0 .01729 .00493 .286 .01236 .715 .01054 .609
A.   337 Extra Dense Flint 1.6469 33.7 .01917 .00541 .285 .01376 .720 .01170 .655
A.   299 Densest Flint 1.7129 29.9 .02384 .00670 .281 .01714 .789 .01661 .678