On the Visibility, &e. By J. W. Ste])henson. 567 



That although the full aperture and resolving power are 

 secured by mounting in balsam, it gives nevertheless nearly the 

 faintest image of all. 



That a solution of phosphorus is, as far as visibility is con- 

 cerned, by far the most effective, but the difficulties attending its 

 use must render it unpopular. 



The next best is a solution of sulphur in bisulphide of carbon 

 (although pure bisulphide is very good), and with these there is 

 no technical difficulty whatever. 



A ring of the aqueous solution used by Mr. Browning in 

 making his bisulphide prisms being formed on the slip, and a drop 

 of the sulphur solution or pure bisulphide being placed in its 

 centre, nothing is necessary but to place over it the thin cover 

 with its adhering diatoms, press it down on the still moist ring, 

 running round it a somewhat copious margin of the cement, and 

 the thing is done. 



In a short time the glutinous cement sets and finally becomes 

 dry, when, in order to protect it from the water of the ordinary 

 immersion lenses, it is desirable to give it a coat of gold size, or 

 shellac varnish, although for mere keeping purposes this is un- 

 necessary. 



The same course may be adopted in mounting in phosphorus, 

 except that the solution must be run in from the edge of the thin 

 cover to avoid the phosphoric acid which rapidly forms on its 

 surface, and destroys the effect wherever it comes in contact with 

 the object. I have found varnish made of the best red sealing- 

 wax (which is better than pure shellac) as useful as Browning's 

 aqueous cement above referred to, but as it is brittle when dry it 

 should also be protected by a coating of gold size. 



There are now on the table objects mounted in phosphorus and 

 bisulphide of carbon, which I exhibited in 1873,* and they still 

 remain unchanged notwithstanding the volatile nature of the 

 materials. On that occasion I fell into the error of saying that 

 there was a loss of aperture (instead of angle) with dry objectives 

 on objects mounted in phosphorus and bisulphide of carbon, when 

 in fact the aperture remained unchanged. 



* See 'Mon. Micr. Journ.,' x. (1873) p. 1. 



