dorsey: luminescence measurements 3 



Both ends of the box are closed, but in the front end is set a 

 milk-glass window 2 or 3 cm. square, backed by a color filter 

 so chosen as to make the transmitted light match in color the 

 luminescent light that is to be measured. In the bottom of the 

 front end of the box there are suitable openings through which 

 pass the cords for operating the carriage, the steel tape, and the 

 leads carrying the current to the lamp. The entire interior of 

 the box, including the ways and the lamp carriage, is painted a 

 dead black, and movable black diaphragms and screens must be 

 so placed as to screen the milk-glass window from all light 

 scattered from the walls of the box. All joints must be light 

 tight, and the openings through which the cords, etc., pass must 

 be suitably screened or provided with black cloth hoods to pre- 

 vent the passage of light. It is desirable that the top of the 

 box be hinged, so as to facilitate the adjustment of the dia- 

 phragms and lamp. 



The photometer is used in a darkened room. As the lumi- 

 nosities with which we are concerned are often quite faint, any 

 stray light, such as may leak under a door, becomes very annoy- 

 ing and should be carefully excluded. In the lamp circuit are 

 placed a suitable ammeter and adjusting rheostats. Unless 

 there is a second observer whose sole duty it is to keep the 

 current at the proper value, the current should be supplied by a 

 storage battery being used at the time by no one else. 



In order to make a measurement, the luminous material, ap- 

 plied to a card or enclosed in a thin walled glass tube 8 of small 

 diameter, may be placed directly against the milk-glass screen 

 and the position of the lamp is then adjusted by the cords until 

 the luminosity of the milk-glass appears to be the same as that 

 of the material. The observer's line of sight should be perpen- 

 dicular to the milk-glass. If the color match is good the tube 

 will almost disappear when the adjustment is correct. The 

 luminosity of the material is inversely proportional to the 



1 The tube should be painted black over two-thirds of its circumference, so as 

 to screen the milk-glass from the light given out by the material. The unpainted 

 side is turned toward the observer and the tube is rotated so as to reduce 

 the visible dark line 'on one side to the smallest amount consistent with a screening 

 of the milk-glass. 



