170 CARNEGIE INSTITUTION OF WASHINGTON. 



signed that the path of Hght-rays cuts the striations in a direction that is as 

 nearly as possible normal to the direction of the striations themselves. 



The methods of manufacture and inspection are described and a discussion 

 given of the manner of forming the glass into blanks for prisms and lenses. 



This method lends itself admirably to large-scale manufacture of glass 

 suitable for most of the optical instruments used in warfare, for photographic 

 lenses, field-glasses, spectacles, and low-precision instruments in general. 

 For instruments of the highest precision, recourse must still be had to the older 

 process of coohng the glass in the pot. 



(14) Note on the motion of the stirrers used in optical-glass manufacture. E. D. Wil- 



liamson and L. H. Adams. J. Am. Ceramic Soc, 3, 671-677 (1920). (Papers 

 on Optical Glass, No. 25.) 



Perhaps the most noticeable difference between the manufacture of optical 

 glass and that of other types of glass (bottle, window, and plate) is that the 

 melt must be stirred vigorously in order to get complete mixing. The reason 

 behind this necessity is the close approach to homogeneity specified in the 

 tests to be passed by the finished article. 



The stirrers used are necessarily of a very simple nature, generally consisting 

 of a clay rod attached at right angles to an iron water-cooled pole, the other 

 end of which is driven at steady speed in a horizontal circle, while supported 

 near the center by a pulley or other support. Such a device does not cause 

 circular motion of the stirring-rod, but causes it to describe an egg-shaped 

 figure at a variable speed. A slightly more uniform motion is obtamed if a 

 pin attached to the pole sUdes in a fixed slot instead of the pole passing over the 

 pulley, but the general results are not very different. If the support is not 

 close to the center of the pole, the motion of the rod is far from circular and a 

 large part of the glass in the pot is not stirred. 



(15) Italite: a new leucite rock. Henry S. Washington. J. Wash. Acad. Sci., 10, 270-272 



(1920). 



A brief statement of the article reviewed under No. 16 below. 



(16) Italite: a new leucite rock. Henry S. Washington. Am. J. Sci., 50, 33-47 (1920). 

 This very exceptional rock is represented by a small specimen from a flow 



on the west slope of the volcano of Rocca Monfina, north of Naples, where it 

 was collected by Baron Dr. G. A. Blanc and Ing. F. Jourdain, of Rome, who 

 kindly gave the specimen and permitted the publication of the results of its 

 study. 



The rock is rather coarse-grained and friable, composed almost entirely 

 (90 per cent) of leucite crystals, 3 to 5 mm., that show the twinning structure 

 remarkably well, with small amounts of aegirite-augite and a titaniferous gar- 

 net, and a little interstitial colorless glass base, which is probably composed in 

 great part of uncrystaUized noseUte and meUlite. The presence of glass and 

 the microstructure show that the rock is from a flow, and is not a tuff. 



The chemical analysis gave: 



SiOo 51.02 K2O 17.94 P2OB 0.02 



Al,63 22.21 H2O + 0.82 SO3 0.76 



FeaOs 1.48 H2O- 0.11 CI 0.08 



FeO 0.57 CO2 none (Ce, Y)203 trace 



MgO 0.14 TiOj 0.57 MnO 0.01 



CaO 2.31 ZrOs 0.06 BaO 0.20 



NajO 1.67 



99.97 



This analysis is extremely remarkable, especially in the high percentage of 

 K2O, which is far above that of any other known rock, the next highest figure 

 being only 11.91. 



