ZOOLOGY AND BOTANY, MICROSCOPY^ ETC. 265 



In his examination of new fluids, Dr. van Heurck met with no 

 sufficient success amongst chemical products, but of vegetable sub- 

 stances three were discovered which appear to be in every way 

 suitable. 



The first is a solution of the resinous gum known as oliban (from 

 several species of JBosweUia of East Africa) partially dissolved in 

 cedar-oil. It gives a fairly thick lemon-yellow liquid of refractive 

 index 1-510, and dispersive power 0-0077. To prepare the liquid, 

 pieces of very pure oliban are powdered finely, and the powder, mixed 

 with its ovnx volume of cedar-oil, is heated in the water-bath in a 

 glass beaker for 2-3 hours. It is then left till the next day, when the 

 supernatant liquid is drawn off. 



The resin (elemi) 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 is obtained with a refractive index 

 of 1-519, and dispersive power of -0074. By adding castor-oil to 

 the solution in suitable quantity the index is lowered to 1-508, and 

 the dispersive power to • 0072. To prepare the solution, 20 parts 

 by weight of the tacamaque are dissolved in the water-bath in 22 parts 

 of cedar-oil and 14 parts of castor-oil added. 



According to Professor Abbe, the latter solution and that of 

 dammar in cedar-oil constitute the two best fluids for homogeneous- 

 immersion objectives. 



The third is copaiba of Maracaibo, derived from Copaifera offici- 

 nalis. That found in commerce at Antwerp, and apparently authentic, 

 had an index of 1-519, whilst a specimen from Guiboiu't of copaiba 

 of Para was only 1-506. It dissolves readily in cedar-oil. Another 

 liquid of 1-510 index and -0076 dispersive power is obtained by dis- 

 solving 7 parts of light vaseline in 30 parts of copaiba. A very 

 thick liquid results, not attacking varnish even after a contact of 24 

 hours. If it is found to be too thick it can be diluted by mixing with 

 it a solution of copaiba in cedar-oil. 



Other liquids from conifers were tried, but in all the dispersive 

 power was found to be too high. 



Dr. van Heurck fears that it will be very difficult to discover any 

 substances which will satisfy microscopists who prefer aqueous 

 liquids. 



Advantage of Homogeneous Immersion.* — Dr. van Heurck also 

 says that "the suggestion of Mr. Stephenson .... constitutes 

 certainly the greatest advance which has been made in microscopy 

 during late years. Personally we have been able to appreciate, better 

 perhaps than any one, the importance of such objectives, for it is 

 owing to them that the thousands of drawings in the ' Synopsis des 

 Diatomees de Belgique ' could be furnished in a relatively short time. 

 "When we think of the trouble that monochromatic illumination has 

 caused us, and the frequent interruptions necessitated by the absence 

 of the sun, we cannot sufficiently congratulate ourselves upon this 

 fortunate discovery, which has enabled us to advance, by a good many 



* Loc. cit., pp. xxii.-iii. 



