560 Scientific Intelligence. 



determined with ease and accuracy. The data also show that the 



assumption of a linear damping term in the equation of motion of 



the iron cylinder was fully justified. By substituting the experi- 



2u 

 mental numbers in the working formula it was found that had 



e 



the value —I'll X 10 " 7 which is in excellent agreement with the 

 accepted datum —1-13 X 10~ 7 . The problem has now been suc- 

 cessfully attacked in converse ways : the above authors varied the 

 magnetization and found the predicted mechanical rotation, while 

 S. J. Barnett (Science, July and October, 1915) imparted a great 

 angular acceleration to a specimen of iron and demonstrated the 

 existence of a magnetomotive force. — Verh. d. deutscli. phystk. 

 Gesell., No. 8, p. 152, and No. 22, p. 420, 1915. H. s. u. 



7. Refraction of X-Mays. — On the simple theory of disper- 

 sion the formula for the square of the refractive index involves 

 the sum of terms of the type A/eV^m - '^ 8 — n 2 )' 1 , where n and 

 n denote respectively the frequenc}^ of vibration of the oscillators 

 and of the incident radiation. As the frequency of the incident 

 radiation increases from n, for visible light, to n' for penetrating 

 X-radiation, each "infra-red term " diminishes approximately in 

 the ratio n /n n and each " ultra-violet term " (including the K, L, 

 and M characteristic-radiation terms) in the ratio —n^/n'". If 

 n' be higher than the natural frequency of any of the constituent 

 electrons all of the corresponding terms will become negative and 

 therefore all the vibrating systems will affect the index of refrac- 

 tion in the same way. Assuming the general validity of this line 

 of argument 0. G-. Baekla deemed it worth while to subject the 

 matter to careful experimental test. 



The radiation from an X-ray bulb was sent through two nar- 

 row vertical slits in parallel lead screens which were about 50 cms 

 apart. The rays finally fell upon a photographic plate at a. dis- 

 tance of about 150 cms from the second screen. The image thus 

 produced was a line 3 cnjs long and about O^Ol " 1 wide. Two crys- 

 tals of potassium bromide, one above the other, were interposed 

 in the path of the beam of X-rays at about 140 CIBS from the plate. 

 The refracting edges of the prisms w r ere parallel to each other and 

 to the plane of the slits, but the so-called bases of the prisms 

 Avere turned away from each other so as to lie on opposite sides 

 of the beam of radiation. Consequently any refraction of the 

 rays arising from transmission through the prisms would cause 

 displacement in opposite directions of the upper and lower halves 

 of the thin pencil of X-rays. An exposure of a fast plate was 

 made for a period of about eight hours. It was found that the 

 photographic impression was still an unbroken straight line. 

 There was no evidence of the slightest relative displacement of 

 the halves of the image. When looking along the line in a direc- 

 tion making a small angle with the plane of the plate it was esti- 

 mated that a lateral shift of 0-025'" m could have been detected. 

 From the fact that transmission through a prism did not produce 

 a deviation as great as 2 seconds of arc, Barkla concludes that the 



