158 



SCIENCE 



[Vol. LV, No. 1415 



The instrument was of tlie type based upon 

 the well known "thermo-gauge" devised by the 

 late E. F. Morse for controlling the tempera- 

 ture to which metals are heated in tempering. 

 It consists of a telescope of short focus, low 

 power and relatively large aperture, having a 

 tungsten lamp in the focal plane. The fila- 

 ment of this lamp is superimposed upon the 

 image of the surface the brightness of which 

 is to be measured. With a screen in the eye 

 piece so selected as to give a fair color match 

 with a minimum of absorption it is easy to 

 adjust the current through the lamp until the 

 filament merges in the surface with which it 

 is to be compared. The lamp may be com- 

 pared for temperature as in ordinary optical 

 pyrometry or directly for brightness, using an 

 illuminated matte surface subject to known 

 fluxes of light. 



While this scheme becomes rigorous only in 

 the comparison of non-selective radiation, the 

 departures are not troublesome. Nearly all 

 fluorescence colors, on account of the broad- 

 banded character' of their spectra, may be 

 regarded as modified whites. They are in 

 general of diluted rather than of saturated 

 hue. Again, in dealing with very low intensi- 

 ties, as in the study of luminescence, slight 

 color differences become quite inappreciable. 

 All that is demanded of the color screen is to 

 give a ruddy, greenish or bluish tone respec- 

 tively so as to avoid strong contrasts. 



Fortunately, as in all photometric processes 

 involving the distinguishing of a pattern, the 

 sensibility of the determination does not fall off 

 seriously with the diminution of the field of 

 view until it becomes difiicult to distinguish 

 outlines. 



With this apparatus the breaking down 

 oceiu's at very low intensities. Any surface 

 against which the unlighted filament can be 

 seen as a black line can be measured as to 

 brightness with surprising consistency. In 

 searching for a white surface devoid of 

 luminescence, for example, it was easy to 

 detect traces of fluorescence in a variety of 

 substances usually deemed inactive. Thus 

 Becquerel,^ more than sixty years ago, noted 

 that viewed in his phosphoroscope nearly 



^Becquerel: La Liimiere, Vol. 1, p. 256. 



everything non-metallic glowed. The only 

 point here is that by this simple method these 

 traces are found to be measurable. 



Because of its availability at very low inten- 

 sities this instrument is likewise adapted to the 

 determination of persistent phosphorescence. 

 One has only to find a suitable screen, focus 

 the pyrometer on the phosphorescent body, 

 note the cessation of excitation on a chrono- 

 graph and record thereafter the times at which 

 the phosphorescence matches the filament 

 which is set successively to a diminishing series 

 of predetermined brightnesses. 



The accompanying table gives estimates of 

 the brightness of a number of fluorescent 

 materials determined in this way. The excita- 

 tion was approximately the same for all, ex- 

 cepting in the case of luciferin. For a sample 

 of this interesting substance, made from 

 marine light-giving organisms, I am indebted 

 to Professor E. N. Harvey. The luciferin was 

 activated by wetting the powdered material 

 and stirring vigorously to hasten oxidation. In 

 all other cases an iron spark was used at a 

 distance of about ten centimeters. The spark 

 was obtained by means of the convenient step 

 up transformer designed by Mr. W. C. 

 Andrews for that purpose. 



THE BRIGHTNESS OF FLUOEESCENT 



SUBSTANCES 



BRIGHTNESS IN 

 SUBSTANCES MILLILAMBERTS 



Dyestuffs in dilute solution: 



Rhodamin 6 G 4.2 to 12.0 m. 1. 



Ehodamin B 5.2 



Fluorseein 4.2 to 5.2 



Tetraehoreosin 4.2 



Resorufin 3.0 



Luciferin (prepared by Pro- 

 fessor Harvey) 14.5 to 16.0 



Uranyl salts (solid) : 



Potassium uranyl sulphate.... 35.2 



Ammonium uranyl sulphate.. 23.0 



Rubidium uranyl chloride 8.11 



Potassium uranj'l nitrate 7.53 



Uranyl nitrate 6.61 



Caesium uranyl nitrate 5.71 



Uranyl acetate 5.39 



Potassium uranyl fluoride 4.69 



Cffisium uranyl acetate 4.56 



Lead uranj-l acetate — 3.75 



Miscellaneous solids: 



Synthetic willemites 12.5 to 14.0 



Natural willemite (Franklin 



Furnace) 5.31 



Sidot Blendes 3.08 to 10.9 



