CLEAR FUSED QUARTZ BERRY 217 



Its index of refraction for the D line is 1.459, and while its dis- 

 persion is higher than optical glass it is more constant because of 

 the smaller effects due to temperature changes. If the rays which 

 have entered are nearly parallel to a rod of quartz they are totally 

 reflected internally, and on account of this can pass around curves, 

 unless too sharp. This property, coupled with a very small absorp- 

 tion loss, makes it possible to transmit light through very great 

 lengths of curved rod or tubing with very little loss. A rod of this 

 fused quartz 1 meter long will emit at one end about 93 per cent of 

 the total visible light passed into the other end. For the better 

 grades of optical glass the highest percentage transmitted under the 

 same conditions is not more than 65 per cent. 



The ordinary ran of quartz made by this process and used in the 

 fabrication of quartz mercury-arc lamps will transmit light wave 

 lengths as low as the 1850 A line in the ultra violet. From the oppo- 

 site end of the spectrum the heat rays also are transmitted with little 

 loss. For example, if one end of a fused quartz rod 12 inches long is 

 heated to incandescence, it will be found very uncomfortable to hold 

 the finger over the other end, although one may comfortably grasp 

 the rod a few inches from the heated zone. 



Clear fused quartz is the only known material which can be ob- 

 tained in quantity and which is transparent to ultra-violet rays. 

 The medical profession is consequently utilizing this material more 

 and more in its application of ultra-violet light for therapeutic 

 purposes. 



By the process here described it is now possible to make tubes, rod, 

 ribbon, and cane in lengths of 30 feet and in shorter lengths of 

 diameters up to 8 inches. Blocks have been made up to lli^ inches 

 in diameter and 6 inches thick having comparatively few bubbles, 

 and these widely separated, comparable in general appearance to the 

 best quality of optical glass. Such material is suitable for prisms, 

 lenses, and for use in instruments where the visible ray is an im- 

 portant factor. 



Constant progress in this development is being made, and it is 

 hoped that perfect homogeneity can be made the rule rather than 

 the exception. It is expected that for most optical work this quality 

 of the present material will be entirely satisfactory. 



PROJECTION LENSES 



Fused quartz lenses of 4i/^ inches diameter have been for several 

 months very successfully operated on test in motion-picture projec- 

 tion machines, using currents as high as 150 amperes and where glass 

 lenses cracked almost daily. 



