78 The Microscope. 



The refractive index of such a solution can be very approxi- 

 mately calculated when the indices of solid and solvent, and the 

 relative quantities of each are known. 



The laws that govern the refractive indices of chemical com- 

 pounds are not yet well known, so that in most cases it is diffi- 

 cult to foretell the probable index of a given combination, the 

 indices of the elements being known. In some of the organic 

 series, however, considerable progress has been made in this line 

 of investigation. 



Of the ordinary highly refractive media monobrom balsam, 

 purified American styrax, and probably Weir's monobrom tolu 

 will prove the most satisfactory. Monobrom balsam owes its 

 increased index to the monobrom-naphthalin in which the bal- 

 sam is dissolved. The gum of Liquidamber styraciflua, or Ameri- 

 can styrax, is a resin of high refractive index, chloroform or 

 benzol being used to reduce it to proper fluidity. In Weir's 

 medium we have a highly refractive resin dissolved in a highly 

 refractive solvent. 



In preparing media of a resinous nature such as styrax, bal- 

 sam of tolu, etc., great care should be taken to purify them 

 thoroughly so as to extract all products which might separate 

 out^in time and render the mount granular or crystalline. The 

 other media having high refractive indices, such as solution of 

 potassio-mercuric iodide, monobrom-naphthalin, solution of 

 phosphorus in carbon disulphide, etc., are either difficult to seal 

 properly, dangerous to manage or otherwise unsatisfactory. 



Many of the so-called chemical media containing bromine, 

 arsenic, sulphur, chloride of tin, bromide of antimony, etc., are 

 all liable to become granular or crystalline in time, though owing 

 to their high refractive index they give very brilliant effects, 

 while they remain clear. 



NOTES ON THE MICROSCOPE STAND AND ON SOME OF 

 ITS ACCESSORIES. 



AN AMATEUR. 



XVI. 



THE SELECTION, CARE AND USE OF A STAND. 



ON only a slightly lower jilane than the best two stands in the 

 world, the " Congress " of W. H. Bulloch, and the "American 

 Centennial " of Joseph Zentmayer, are Mr Zentmayer's " New 



