1882.] 



MICROSCOPICAL JOUIIAL. 



27 



tion of Juniperis Virginiana. It has an 

 index of refraction varying from 1.505 

 to 1.507. Its dispersion is 0.0073. ^^ 

 has been lately attempted to over- 

 come the objection to the essence of 

 cedar, its excessive fluidity, by dis- 

 solving in it some damar, which at 

 the same time permits its index of re- 

 fraction to be raised to 1.520.* 



Essence of copaiba has the same 

 index of refraction as the oil of cedar, 

 and is somewhat less fluid. It is, 

 therefore, an excellent substitute for 

 the latter. This essence is not fur- 

 nished by the true copaiba, but comes 

 from the distillation of Indian copai- 

 ba, yielded by different species of 

 Diptrocarpus. 



Our first researches were directed 

 among the chemical substances, but 

 they led to no important results. We 

 then turned our attention to the na- 

 tural, vegetable products, and the re- 

 sult has been favorable. The oleo- 

 resins, the resins and the gum-resins 

 of the Terebinthacge, having yielded 

 the best results. 



The balsam of copaiba, already 

 mentioned, has an index of 1.509. It 

 is, therefore, preferable, owing to its 

 viscosity and its higher index, to the 

 essence of the same product. The 

 gum-resin olibanum (yielded by sev- 

 eral Boswellia of eastern Africa), 

 commonly named incense, partly dis- 

 solves in essence of cedar, giving a 

 quite a thick liquid, of which the index 

 of refraction is 1.5 10 and the disper- 

 sive power is 0.0677. 



To prepare this liquid the pure 

 drops of olibanum are finely pulver- 

 ized, and the powder obtained mixed 

 with an equal volume of oil of cedar 

 and heated on a water-bath for two 

 or three hours. Allow the liquid to 

 stand, and when clear decant the su- 

 pernatent fluid. 



The resin of Manilla, the botanical 

 origin of which is uncertain, dis- 



* Prof. Abbe has lately mentioned an ex- 

 cellent liquid, which is obtained by dissolving 

 dan'ar in oil of cedar until an index of 1.520 

 is obtained and which can be brought back to 

 1.509 by an addition of castor oil. 



solves readily in oil of cedar, and thus 

 furnishes liquids of which, according 

 to the proportions of the substances 

 employed, the index may be made to 

 rise from 1.510 to 1.520, with a dis- 

 persive power of 0.0076. , By adding 

 castor oil to the solution a suitable 

 liquid is obtained, having an index 

 of 1.508 and a dispersive power of 

 0.0073. But this solution seems to us 

 less useful than the solution of oli- 

 banum, because it is slightly sticky. 



The resin of Brazil,* and the white, 

 oily tacamaque of Guibourt give 

 equally good solutions with oil of 

 cedar. By dissolving the tacamaque 

 in the oil, a liquid having an index of 

 1.5 19 and a dispersive power of 

 0.0074 is obtained. By adding to the 

 solution castor oil in suitable quantity, 

 the index descends to 1.508 and the 

 dispersion to 0.0072. 



To prepare the solution, dissolve, 

 on a water bath, 20 parts by weight 

 of the resin in 22 parts of the oil of 

 cedar, and add 14 parts of castor oil. 

 According to Prof. Abbe, this 

 solution, and that of damar in oil 

 of cedar, are the best ones known for 

 homogeneous-immersion objectives. 



An authentic specimen cf Chinese 

 varnish, but already possessing sap of 

 Rhus verni, L., brought from China 

 by Perrottet, has an index of 1.527. 

 This oleo-resin mixes perfectly with 

 the essence of cedar, giving solutions 

 which may be employed. The tur- 

 pentine of Scio .is furnished by the 

 Pistacia terebinthus. We have found 

 1.535 as the index of refraction of a 

 sample of old and pasty resin, from 

 the collection of Guibourt. It dis- 

 solves in oil of cedar and may give a 

 liquid of the desired index. 



Copaiba is furnished by Copaifera 

 officinalis, L. That found in com- 

 merce at Antwerp, and which appears 

 to be the true copaiba of Maracaibo, 

 is of a clear brown color, and has an 

 index of 1.519, and an authentic 

 specimen, from Guibourt, of copaiba 

 of Para has only an index of 



* Resin of Fagara octavdra. 



