ZOOLOGY AND BOTANY, MICROSCOPY^ ETC. 319 



impurities, probably owing to the rough method employed in obtain- 

 ing it — the stems are cut in small pieces and boiled, when^ the gum 

 rises to the surface, and is skimmed off; and second, from its thick- 

 ness. It is therefore necessary that it should be dissolved in one of 

 the following menstrua : chloroform, benzol, ether, a mixture of 

 benzol and absolute alcohol. When the resin is dissolved it must be 

 filtered, and it is then ready for use. The solution should be _ of the 

 colour of brown sherry, and the consistency of limpid olive oil. Its 

 consistency can of course be increased by evaporating a portion of the 

 benzol, and the whole of the latter should be eliminated before 

 placing the cover-glass on the slip. Its refractive index is then 1 • 63, 

 very nearly that of monobromide of naphthaline. The American 

 liquid-amber is prescribed in the American Pharmacopoeia, but seems 

 to be unknown in Europe. It would, if obtainable, be preferable to 

 gum styrax, as its colour is a pale yellow. The colour of the styrax 

 is practically of little consequence, as the film between the cover and 

 slip is very thin, and does not show any appreciable amount of colour 

 when placed under the Microscope. 



During the past four or five months Mr. Eitton has used this medium 

 for various Diatomacete. The transverse stride on Pleurosigma littorale 

 and the longitudinal on Navicula cuspidata are much more sharply 

 defined, and the stria3 on all of them are more easily resolved than when 

 mounted in Canada balsam. The most striking difference between gum 

 styrax and Canada balsam is displayed by Polymyxus coronalis. In 

 balsam, the valves are perfectly hyaline, and the rays and puncta almost 

 invisible ; in gum styrax the valves are light brown, and the markings 

 easily resolved. Heliopelta, as might be expected, does not exhibit 

 more structural detail, but every line and dot is more distinct than 

 when it is balsam-mounted. Several of the Aulisci are also much 

 improved when mounted in this medium. Mr. Kitten cannot say 

 much of its merits as a medium for mounting other microscopic 

 objects. He has tried it for thin wood sections, hairs, chalk 

 foraminifera, and a few butterfly scales, all of which show better than 

 they do in balsam. The colour of styrax becomes objectionable when 

 a thick layer is necessary. Dr. Van Heurck directs that the com- 

 mercial gum styrax should be exposed in thin layers to the light 

 and air for several weeks, to eliminate the moisture contained in it 

 previous to dissolving it, but Mr. Kitten has not found this necessary 

 with his sample. 



Mounting Medium of High Refractive Index.* — Professor 

 Hamilton Smith is reported to have mounted Ampliipleura pellucida 

 and Navicula rhomhoides in " something having a refractive index of 

 2 • 4," the result being " past all expectation, beating everything yet 

 seen," " making a new era in diatom mounting," and " far surpassing 

 all that has been done in phosphorus." 



Dr. A. Y. Moore has also "f mounted A. jpellucida in a medium of 

 index 2 • 3. ^^ The appearance of the frustule is said to be " quite 



* Jouni. Quek. Micr. Chib, i. (1884) pp. 333-4. 

 t Amer. Mou. Micr. Journ., v. (1884) p. 37. 



