EXAMINATION AND PRESERVATION MEDIA. 



235 



tbe optical effects oi' various media. The figures give the 

 approximate indice." of refraction. They should be accepted 

 with some caution, on account of the variability of samples. 

 The figures given for balsam refer evidently to the resin in the 

 solid state and not to the solutions used for mounting, which 

 are certainly mucli lower, according to the lower index of 

 the solvent. 



It will be seen that cedar oil has nearly the index of 

 crown glass (this is true of the oil in the thick state to which 

 it is brought by exposure to the air — not of the new, thin 

 oil, which is less highly refractive) ; it therefore clears to 

 about the same extent as Canada balsam. Clove oil has a 

 nuich higher index, and therefore clears more than balsam; 

 cinnamon oil higher still. Turpentine and bergamot oil have 

 much lower indices, and therefore clear less. 



Wutery Media. 



401. Isotonic and "Indifferent" Liquids.— The old distinction 

 of "Indifferent" liquids, and those which have some action 

 on tissues, appears to be miKleading more than helpful ; for 



