1-01. 2 , s o . 2
134 Conclusion and Acknowledgment
Literature Cited
The apparatus described has performed satisfactorily in the Bell Telephone Laboratories. -4commercial modification has been in successful operation for a year. It is, however, somewhat in the laboratory stage of development as yet, and a considerable amount of work is necessary in order to make the apparatus suitable for general plant use. I n this connection, the kindness of the Bell Telephone Laboratories, and particularly A. C. Walker, must be acknowledged, as calibration of the apparatus would have been very difficult without the calibration apparatus which n-as developed by Doctor Walker.
(1) Behr, J . O p f i c a l Soc. Am., 12, 623 (1926!. (2) Daynes, Proc. Roy. Soc. ( L o n d o n ) , 9TA, 273 (1920). (3) Grifiths, Proc. Phys. SOC.Loitdon, 34, v-xcii (1921-22:i. (4) Gruss and Schmick, W i s s . I'er8fenllich. Siemzxs-k'onaern. 7, 202 (1025,. ( 5 ) Koepsel, Ber. p h y s i k . Ges., 10, 814 (1908); 11, 237
