234 COLLECTION AND PREPARATION 



necessary to take up the time of those who have read it, but 

 will simply add that I have almost discarded Balsam in 

 Benzole, and use the Styrax solution in preference. I have made 

 a series of experiments with some sixteen other media, but none 

 have proved so satisfactory for cleanliness and clearness of 

 resolution. The refractive index of Styrax is, I believe, much 

 the same as that of Monobromide of Napthaline. 



Some little attention has been drawn of late to the mounting 

 of diatoms in a solution of Bmiodide of Mercury and Iodide of 

 Potassium ; but whilst the image is without doubt somewhat 

 sharper than when mounted in Balsam, still, the possibility of the 

 breakage of the cover-glass, and the consequent escape of the 

 mercurial solution over the objective and the brass work of the 

 microscope seems hardly worth the risk, especially when it is 

 borne in mind that the refractive index of styrax is even higher 

 than that of the fluid just named. Canada balsam has an index 

 of 1*54, the solution of Mercury and Potassic Iodide, i*68 ; next 

 follows Styrax, and finally Phosphorus, with that of 2'i. 



A few remarks on the latter medium may not be out of place, 

 both as regards its solution and the method of using it. It is 

 necessary to procure clean, semi-transparent Phosphorus, and 

 having cut off, under water, some large pieces with a pen-knife, 

 place them for a few seconds on a piece of white blotting-paper to 

 free them from the slightest suspicion of water. Before com- 

 mencing, the operator should be provided with a small basin of 

 water in which to place any article that has been touched with the 

 phosphorus solution to prevent accidental combustion. And as 

 this medium is liable to oxidation, it is better to make but a small 

 portion at a time — that is to say, one drachm of Phosphorus to 

 two drachms of Bisulphide of Carbon. When the former 

 is quite dissolved, slightly damp a piece of filtering paper 

 with Bisulphide of Carbon, and with a very small glass funnel 

 placed in the neck of a stoppered bottle carefully filter the solu- 

 tion. Place the glass funnel and the filtering paper, when used, in 

 the basin of water to prevent accident. Supposing the diatoms 

 are preserved in a small tube of water and spirit, all that is required 

 is to place a drop of the fluid on the cover-glass, and slowly eva- 

 porate the medium over the flame of a spirit-lamp or jet of gas. 



