Richardson's handbook of projection (1927)

830 HANDBOOK OF PROJECTION FOR The Bausch and Lomb Optical Company were the first to find a really satisfactory backing, which consisted of either fireclay or a composition closely resembling it. It was sprayed on in semi-liquid form, over the silver coating on the mirror back. Two or more coats were applied, and each separate coat was baked at a temperature considerably in excess of the temperature the mirror would be called upon to withstand in ordinary practice. Since the introduction of this backing, trouble with chipping, peeling and disintegration has gradually been reduced until now, from all reports, it is a negligible quantity. WARNING. — It is not unlikely that, if the practice has not already appeared, mirrors for reflector arc lamps will be placed on the market by irresponsible parties. In purchasing goods of this kind it is extremely unwise to consider anything not bearing the trade mark of a responsible, thoroughly reliable business institution — an institution which will make a reasonable guarantee and back it up. If you do buy goods of this kind which do not bear such a trade mark, you are inviting trouble, and deserve no sympathy if you get it. CRATER POSITION.— The positive carbon should be centered exactly on the optical axis. This position may be checked by advancing the positive carbon and making sure that it is exactly central in the central mirror opening. There are two types of crater in use, both illustrated in E — #HE— z=d f »£, -f Figure 315A Fig. 315A, in which the blunt nose crater is illustrated at A and the angle crater is shown at B. These two types of crater of course result directly from the setting of the negative carbon tip with relation to the positive carbon tip. The one shown at B is not a good crater. The one above at A is an ideal crater, and one which, if properly