146 



NATURE 



[April 24, 19 19 



mination (Nature, June 21, 1917)- The prism 

 system is rotated by means of the index arm until 

 a dark shadow comes into the field of view of the 

 telescope, and the edge of the shadow is adjusted 

 exactly on the cross-lines. The refractive index 

 for sodium light is then read directly on the scale 

 of the instrument, the accuracy of reading being 

 one or two units in the fourth decimal place. 

 When white light is employed, the dispersion of 

 the emergent light is neutralised by means of an 

 Abbe compensator. It is satisfactory to find that 

 British firms have produced instruments which 

 are undoubtedly superior to the German pattern, 

 and that they have been able to supply the demand 

 in various Government Departments that has 

 arisen during the war. The firm of Adam Hilger 

 now produces standardised instruments in which 

 not only the mechanical, but also the optical, parts 

 are interchangeable (Fig. i). Tables of refractive 



Fig. I. —Abbe Refractomci^-r (Adam Hilger, Ltd.). a, Upper pnsin 

 jacket ; b, lower pri-ni jacket ; i., reader arm ; i , reader ; 

 G, scale ; h, scale arm ; j, telescope ; k, telescope eyepiece ; 

 1., milled head for Abbe compensator ; M, scale for Abbe com- 

 pensator ; N, adjusting ring for lower D.V. prism; o, mirror; 

 (,i, s, and T, prism jacket nozzles; v., nozzle with thermometer 

 chamber. 



indices of industrial substances are in course of 

 publication, and should prove of great value. 



Messrs. Bellingham and Stanley have produced 

 an instrument of distinctive design, embodying 

 several improvements on the German type (Fig. 2). 

 The prism-box is now designed to open away 

 from the operator, which makes it much easier to 

 examine plastic or solid substances. It is no 

 longer necessary to reverse the instrument, and 

 NO. 2582, VOL. 103] 



full use may be made of the illuminating mirror. 

 At the same time, the change permits of greater 

 rigidity of construction. The reader arm is pro- 

 vided with a slow motion by a simple friction 

 device, and the halves of the prism-box may be 

 separated automatically by a small movement of 

 the clamping head. The lower half is so con- 

 structed that it can be removed quickly without 

 tools. 



(Bellingham and Stanley, Ltd.). 



When measurements of a higher degree of 

 accuracy are desired than is possible with 

 the Abbe type • of instrument, the dipping 

 refractometer (Fig. 3) may be employed, but 

 with a single fixed prism readings can be 

 obtained only over a limited range of re- 

 fractive index. The prism of the instrument 

 dips into the liquid, which is placed in a small 

 containing vessel, and the refractive index is 

 determined by the position of the border-line of 

 total reflection seen in the eyepiece. The eyepiece 

 carries a photographic scale, and a micrometer 

 screw adjustment is provided whereby the posi- 

 tion of the border-line with respect to the scale 

 division may be measured directly. A table is 

 supplied giving the refractive index in terms of 

 the scale reading. In the German type of instru- 

 ment the prism is cemented into its holder, and 

 can be used only for solutions of refractive indices 

 between 1-325 and 1-367. In hot and moist 

 climates the solution frequently creeps up through 

 the cement on to the upper face of the prism. 

 When this occurs, or when the prism is damaged, 

 it is necessary to return the entire instrument to 

 the makers. Messrs. Bellingham and Stanley 

 have improved the design of the instrument, and 



