

On Magnetic Double Refraction of Gold Colloids. 353 



double refraction here appears at such weak fields that the 

 direct influence is often of a lower degree of magnitude and 

 can be neglected. 



In all the cases mentioned above, we are concerned with 

 ferromagnetic or strongly paramagnetic solutions. My own 

 investigations include several colloids, among ihem dia- 

 magnetic ones as well. It is above all the irreversible 

 colloids that acquire double refraction in a magnetic field. 

 The degree of dispersity corresponding to a minimum effect 

 differs considerably. Gold particles of the size* 20 fifju for 

 instance, show an evident effect, while the critical diameter 

 of the silver sols is much greater. With other samples it is 

 necessary to extend beyond the limits of the colloidal 

 solutions and to pass to the coarse suspensions in order to 

 find any influence. 



In this paper I give an account of my experimental 

 researches concerning the double refraction of gold colloids, 

 and, finally, I discuss some conclusions as regards the structure 

 of the particles. In a further communication I will consider 

 the optical results. 



In order to measure the phase difference I used Brace's 

 method f. 



Description of the Apparatus. — The lens B (fig. 1) throws 

 an image of the arc A on the rectangular slit C, and this 



Fig. 1. 



B 



C 



D E 



P 



1 F 



G 



H 



S CD 



L ft H O P 



image is projected on the objective of the telescope P by 

 the lens D. The light is passed through the monochromatic 

 filter E, a polarizing Thomson-Glan prism F, the tube Gr, 

 filled with the sample, the half shade or sensitive strip M, 

 the mica compensator mounted on a graduated circle N, the 

 analysing prism 0, and finally the telescope P. The tube G 

 is placed between the poles of the electromagnet H. The 

 principal sections of the prisms were crossed, and form an 



* Equivalent diameter, cp. infra. 



t Phys. Rev. xviii. p. 70 (1904) ; xix. p. 218 (1905). Phil. Mag. [6] 

 yii. p. 317 (1904). Cf. also a Berg-holm, " Doppelbrochung in kathoden- 

 zerstaubten Metallschicliten," Uppsala Universitets Arsskrift, i. (1915). 



Phil Mag. S. 6. Vol. 42. No. 249. Sept. 1921. 2 B 



