Letters Sent to Mr. Markley [E.A. Markley], no date

Dr. Paul D. Berrisford, 810 Lowry Bldg., St. Paul, llinn. Dear Dr. Berrisford: Dr. Lind has written me relative to your interest in the use of radium luminous disca as a weak light source. Dr. Lind has ably advised you relative to the physical charactaristics of our luminous compound. The light intensity of this material decreases with time, and while it is true that the higher grades of our Undark self luminous naterial has a nore rapid decrease than the lower grades, still I doubt if our lower grades would present a uniform enough source of light to be of much value in your work. Even the material whech has stood for a considerable period of tine and had an opportunity to age, so to speak, would still have an appreciable decay curve. I infer from Dr. Lind's letter that you are interested in a fairly constant weak light source, and if such is the case there is considerable question in By mind regarding the feasibility of using radium luminous compound. I an, however, having our Bechanical Department paint up a few disos which we will forward to you during the next few days under separate cover, for your experimentation. The intensity of light from the lower grade dise will probably remain quite constant over a short period of time, but after soveral months will show some change. In connection with problem presented, I am wondering if the best solution would not be to utilize a very woak light source such as may be obtained with the proper set up of resistance etc. In this connection we have designed a pince of equipment for reading our luminous matorial which might very well be adapted to your use. The intensity of the light from this apparatus is of the order of magnitude, naturally of the luminous compound. The light intensity may be veried anywhore from one to 250 microlamberts. This apparatus we have galibrated against a known light source and when in use it would be necessary to check this calibration oceasionaly on account of deterioration of the fildments in the lamp. This would not be difficult, however as the apparatus could be supplied with accessory equipment so that it would be possible to have the calibration checked by a suitable laboratory with- out interforring with the daily uso of the equipment. If you should be interested in this type of equipment I would be glad to go into the matter in more detail with you. Very truly yours, HHBarker:Ri Vice President.