678 PROCEEDINGS OF THE AMERICAN ACADEMY, 



As a standard of radiation was chosen the " black body " ^ formed by 

 the small opening of a cavity kept at 100° C. by a steam jacket. At 

 50 cm. distance this standard radiator, 3.14 sq. cm. in area, produced 

 a deflection of 111 mm., the platinum of the instrument being in the 

 normal to the radiating surface ; and by using Kurlbaum's ^^ measure- 

 ment of the total net radiation from such a body to its surroundings at 

 the room temperature (0.0156 gm. cal. per sq. cm. per sec, distributed 

 through the half sphere according to the cosine law), it was computed 

 that a small source 1 meter away, radiating energy according to the 

 cosine law, must emit altogether 0.00177 gm. cal. per sec. to produce 

 a deflection of 1 mm. In other words, a deflection of 1 mm. was 

 produced -when the platinum was receiving energy at the rate of 

 5.63 X 10~* gm. cal. per sec, per sq. cm. of its surface. 



With this result, an approximate measurement of the total energy 

 of the Rontgen rays was readily obtained. The tube was placed so that 

 the centre of the target was 20 cm. from the platinum of the instrument. 

 A screen of platinum 0.0026 cm. thick was arranged so that it could 

 be interposed in the path of the rays or withdrawn at pleasure. (A 

 screen 0.0014 cm. thick, like the platinum used in the instrument, 

 would have simplified the calculation, but none was available.) Then 

 readings of the throws produced by the rays with the screen out were 

 taken, alternating with readings with the screen in. These readings 

 are given in Table II. In each case the rays were kept on until the 

 maximum deflection occurred, a period of about 10 seconds. From the 

 mean deflection obtained when the screen was not interposed, 0.890, 

 the total energy of that portion of the rays which was absorbed was 

 found to be 



2 TT X 20- X 8.90 X 5.63 X 10"^ gm. cal. per sec, 



or 0.00126 gm. cal. per sec. To find what fraction of the whole radia- 

 tion this represented, it was assumed ^^ that the fraction of rays trans- 

 mitted by a piece of platinum is given in terms of its thickness, t, by 

 g-tt_ Then k was found from 



^•^''* __ -0.0026 i 



0.890 



to be equal to 453. Hence the fraction of the incident rays which the 

 instrument absorbed was 



1 g— *53 X 0.0014 



9 Wicn and Lumrner, Ann. d. Tlivs., 56, 4')! (1895). 

 " F. Kurlbaum, Ann. d. Phys., 65, 74«i (1808). 



^^ Tliis assumption of an exponential absorption law is without justification, 

 and probably in the present case is far from tiie truth. 



