318 



This observation of Stekns seems not to be applicable in a coni- 

 parison of small and large adults. 



Resuming my results, 1 may slate the following conclusions : 



l^K Li the spinal cord of large individuals of the same order the 

 relation of the white substance to the grey is much more in favour 

 of the white than in small individuals. 



2'"l This rule holds good for all the classes and orders examined. 



3''^. This phenomenon can be explained — and according to my 

 opinion has to be exphiined — by the explication given by Ernst 

 DE Vries (vide supra). 



4^'' . Also the shape of the grey mather shows a moditication in 

 large animals in so far as it becomes more gracile, which is 

 chiefly demonstrated by the distance of and the grey connection 

 between the horns. 



Physics. — "7Vie uiacpieto-optlc K]iRR-e ff'ect in ferwmaanetic com- 

 pounds." IV. By Pierre Martin of Geneva. (Communication 

 from the BosscHA-Laboratory by Prof. H. du Bois.) 



In a former paper ^) I have given the dispersion curves of the 

 KERR-effect for certain manganese and iron compounds. It should be 

 particularly noticed that almost all of these curves, as has been 

 further confirmed with new material, show an algebraic maximum 

 and minimum between or near the limits of the visible spectrum. 

 In the present research I have determined some new dispersion- 

 curves and have also investigated in the case of a few substances 

 the relation between the KERR-etfect and the temperature. 



For this pui'pose the material was cut into small discs of the 

 Baine. size as the pole-top (V, 7 mm.) face, and this entire system 

 completely insulated from the pole-shoes by a layer of asbestos. 

 The heating was accomplished by means of an ordinary Bunsen 

 burner. Up to 300^ the temperature was measured with a thermo- 

 meter imbedded close to the mirror. For higher temperatures up 

 to 450° a compressed nitrogen thermometer was used. 



By regulating the flame the temperature could be kept constant 

 to within about 5°. In order to prevent as much as possible 

 oxydation and "tarnish colours" the mirror was constantly bathed 

 with carbon dioxyde which had been led over KMnO^, SnCl^, 



1) P. Martin. These Proceedings 15, p. 138, 1912. 



