Fig. 165
Fig. 166
Fig. 167
The more modern method is to use prism tile set in the necessary frames. The prism receives the direct light from the sky on the upper face and turns, or refracts, it back into the basement, as illustrated in [Fig. 166]. Where the head-beam or other obstructions interfere with the deflection of light direct from the prism, it becomes necessary to use in connection with the skylight or canopy just described an independent vertical frame of window prisms of varying angles. These prisms intercept the light from the pavement tiles and project it into the basement, thus utilizing the light to the maximum advantage.
169. Glazier’s Points.—A necessary adjunct in glazing with putty is the sheet-zinc, triangular glazier’s points, as illustrated in [Fig. 167]. These points are made in six sizes, which range from No. 000, the largest, to No. 3, the smallest. They are also made in somewhat similar shape, called sharps, but this type is not so popular as the triangle point.
170. Grades of Putty.—Putty is usually graded as follows: Commercial, a very cheap grade; pure, a medium grade; and strictly pure, the best grade, which is made of pure whiting ground in linseed oil. The cheaper grades are made in the same manner, but with an amount of adulteration, either in oils or whiting, commensurate with the cost. Therefore, the cheaper the putty, the more it is adulterated, and none but strictly pure should be used when good results are desired.
White-lead putty is in a class by itself, and costs about twice as much as strictly pure whiting putty; but where permanent glazing is required, its use will show handsome returns on the investment.