APPENDIX 2. 



ON PHOSPHOROSCOPES. 



The uranium salts have exhibited under photo-excitation a type of phos- 

 phorescence which persists but a few thousandths of a second, while under 

 cathodo-excitation the type endures for several minutes. It was necessary 

 to devise two phosphoroscopes of entirely different design to measure the two 

 types of phosphorescence. The choice of a suitable phosphoroscope is a 

 matter of great importance ; hence, a brief summary of the types of phosphoro- 

 scopes which have been employed since the time of the great pioneer student 

 of phosphorescence, E. Becquerel, follows. Among the considerations which 

 present themselves when the construction of a phosphoroscope is contemplated 

 are the quantity of phosphorescent material available, the total time of decay, 

 the temperature at which the specimen is to be studied, the nature of the ex- 

 citation (e. g., photo-, ultra-violet, cathode rays, etc.), the ease with which 

 saturation is obtained, and the initial brightness of the specimen. The 

 phosphoroscopes which have been constructed can be divided into three 

 classes : 



Type 1. The specimen is periodically excited and periodically viewed at a 

 later phase. 



Type 2. The specimen is continuously excited and continuously viewed at 

 a later phase. 



Type 3. The specimen is excited for a measured interval of time and the 

 intensity measured at a later time. This method is applicable to the slowest 

 types of decay. 



Machines which may be classified as belonging to type 1 must operate at 

 such speeds that no flicker is noticeable; hence the weakest intensity measured 

 must fall within a total time of decay of one-sixteenth of a second. Many 

 natural crystals, under photo-excita- 

 tion, present very interesting phos- 

 phorescence processes which appar- 

 ently cease in less than this time. 

 The very first steps in the long-time 

 decays of such substances as the 

 natural and artificial sulphides may 

 be studied with the aid of a phosphoro- 

 scope belonging to type 1. E. Bec- 

 querel 1 devised, among other forms, a 

 phosphoroscope of the intermittently 

 excited type. The specimen was 

 mounted between two parallel disks 

 and was alternately illuminated and 

 observed through properly adjusted FlG - 1 - 



openings in the disks. Figure 1 shows two such disks, each having four 

 open sectors, mounted on the same axis but in different phase. Bec- 

 querel caused the exciting light to pass into the translucent crystal through 



1 E. Becquerel, Annales de Chimie et de Physique (3), 55, p. 5. 1859. 



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