As to working field the reflector as ordinarily proportioned is at a disadvantage chiefly because it works at F/5 or F/6 instead of at F/15. At equal focal ratios there is no substantial difference between reflector and refractor in this respect, unless one goes into special constructions, as in photographic telescopes.
In two items, first cost and convenience in observing, the reflector has the advantage in the moderate sizes. Roughly, the reflector simply mounted costs about one half to a quarter the refractor of equal light grasp and somewhat less resolving power, the discrepancy getting bigger in large instruments (2 feet aperture and upwards).
As to case of observing, the small refractor is a truly neck-wringing instrument for altitudes above 45° or thereabouts, just the situation in which the equivalent reflector is most convenient. In considering the subject of mounts these relations will appear more clearly.
Practically the man who is observing rather steadily and can give his telescope a fixed mount can make admirable use of a reflector and will not find the perhaps yearly or even half yearly re-silvering at all burdensome after he has acquired the knack—chiefly cleanliness and attention to detail.
If, like many really enthusiastic amateurs, he can get only an occasional evening for observing, and from circumstances has to use a portable mount set up on his lawn, or even roof, when fortune favors an evening’s work, he will find a refractor always in condition, easy to set up, and requiring a minimum of time to get into action. The reflector is much the more tender instrument, with, however, the invaluable quality of precise achromatism, to compensate for the extra care it requires for its best performance. It suffers more than the refractor, as a rule, from scattered light, for imperfect polish of the film gives a field generally presenting a brighter background than the field of a good objective. After all the preference depends greatly on the use to which the telescope is to be put. For astrophysical work in general, Professor George E. Hale, than whom certainly no one is better qualified to judge, emphatically endorses the reflector. Most large observatories are now-a-days equipped with both refractors and reflectors.
[CHAPTER III]
OPTICAL GLASS AND ITS WORKING
Glass, one of the most remarkable and useful products of man’s devising, had an origin now quite lost in the mists of antiquity. It dates back certainly near a thousand years before the Christian era, perhaps many centuries more. Respecting its origin there are only traditions of the place, quite probably Syria, and of the accidental melting together of sand and soda. The product, sodium silicate, readily becomes a liquid, i.e., “water-glass,” but the elder Pliny, who tells the story, recounts the later production of a stable vitreous body by the addition of a mineral which was probably a magnesia limestone.