636 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



improved instrument as one of most convenient manipulation. It can 

 be used with any kind of light, and is self-reading, i.e. the refractive 

 index (1*3 to 1*7) can be read off on the divided circle without calcu- 

 lation. A very few drops of fluid suffice for observation, and therefore 

 the instrument can be used in the most delicate investigations. Its 

 excellence for liquid observations with transmitted light has long been 

 admitted, but the new form of prism permits of the application of 

 reflected and obliquely incident light (figs. 165-167), by means of which 

 observations can be made on solid bodies. For this latter purpose the 

 telescope is placed approximately vertically, and the frosted surface 

 illuminated by the mirror E (fig. 168). 



Fig. 165. 



Fig. 166. 



Pig. 167. 



Another improvement consists in the addition of a so-called compen- 

 sator placed in the ray-path between the double prism and the telescope. 

 This renders possible the use of white light, and consists of two Amici 

 prisms (exactly adjusted for the D line) of equal dispersion, which by 

 rotation at M can be equally turned about the telescope axis. The 

 dispersions at the two prisms coalesce, and form a resultant dispersion 

 which causes a colourless ray exactly incident on the same spot as the 

 ray from a sodium light. Fig. 168 shows the earlier form of the 

 refractometer fitted with the new compensator. 



The instrument can also be procured fitted with a heating apparatus. 



Zeiss' Educational Refractometer. — This form of instrument is 

 intended for use in physical and chemical laboratories, and depends 

 ou the properties of a flint glass' prism (/x = 1*72) of 61° refractive 

 angle. Its claim to its title is justified by the fact that all the necessary 

 odtical constants (the prism angle and the refractive index of the prisni) 

 can be determined on the instrument itself. Also the apparatus is 

 provided with adjustments by assistance of which the whole of the various 



