624 Prof. E. W. Wood on Cyanine Prisms and a 



§ IX. The Microstructure of the Alloys. 



It has been suggested in Part I. that the general behaviour 

 of the alloys might be explained on the assumption that they 

 consisted of crystals surrounded by a solidified mother-liquid. 



With a view to ascertaining if this was the case the spe- 

 cimens, after grinding and polishing, were etched with dilute 

 nitric acid and examined microscopically. The eyepiece of 

 the microscope was then removed, and the adjustment was 

 altered until the image fell on the screen of a camera. 



When as clear an image as possible was obtained on the 

 screen, a slow plate was exposed for abont 15 minutes. 



In this way photographs (PI. VI.) were obtained showing 

 the forms of the crystals in the specimens containing 5*^4, 

 9*89, and 18*47 per cent, of aluminium respectively. 



Photograph A. (Specimen containing 5*44 per cent, 

 of Al.) Crystals can be seen in the form of Maltese crosses. 



Photograph B. (Specimen containing 9*89 per cent, 

 of Al.) In places crystals having the appearance of a number 

 of rods placed side by side can be seen. Generally the rows 

 are arranged in pairs, the distribution being not unlike that 

 of the bones of a herring. 



Photograph C. (Specimen containing 18*47 per cent, 

 of Al.) The crystals are similar to those shown in photo- 

 graph B ; but are larger in size. 



The magnification of A and C is about 25 diameters. The 

 magnification of B is about 50 diameters. 



An investigation on the magnetic properties of pure alloys 

 of iron and aluminium is now in progress, and we hope very 

 shortly to publish an account of some of the earlier experiments 

 en these purer specimens'*. 



University College, Nottingham, 

 June 7th, 1900. 



LXI. On Cyanine Prisms and a Neio Method of exhibiting 

 Anomalous Dispersion. By R. W. Wood |. 



IH AVE already described a method of making prisms of 

 solid cyanine by pressing the fused dye between plates 

 of glass, which are far superior to liquid prisms or the solid 

 prisms made by Wernicke for the purpose of exhibiting 

 anomalous dispersion. 



Until quite recently I considered that twenty or thirty 

 minutes was about as large an angle as could be used to 

 advantage. With such large angles very little green light 



* Vide Phil, Mag. March 1901. 



t CommuDicated by the Physical Society : read Feb. 22, 1901, 



