On Mounting Objects in Phosphorus. By J. W. Stephenson. 167 



finally completed by two or three successive coatings of gold size, 

 after the first of which, any superfluous glue should be removed 

 with water and a camel-hair brush, and " to make assurance 

 doubly sure " a ring of sealing-wax (shellac) varnish after the last 

 coating of gold size may well be added. 



The slides thus prepared appear to keep perfectly well, as one 

 of P. formosum which I mounted nine years ago and exhibited 

 here on the 4th June, 1873, still remains unchanged ; but it is fair 

 to say, that having been during that period in my cabinet, it has 

 had little exposure to daylight. 



In addition to the increase of visibility there is another point of 

 interest in the use of phosphorus. It was pointed out by Professor 

 Abbe in our last volume, pp. 689 and 832, that depth of vision 



= w( -T7, A- -f- vt — )) from which formula it is obvious that the 



depth of vision (on which stereoscopic vision depends) increases in 

 the same ratio as the refractive index (n) of the mounting medium. 

 Hence it follows that the stereoscopic effect of phosjDhorus, with its 

 index of 2' 1, is more than double that of the same object mounted 

 in air [n = 1), and it is to this circumstance that the stereoscopic 

 appearan-ce of the scales of Machilis maritimus and Tomocerus 

 plumheus under a J^ is to a great extent due. 



There is now another fluid to which it is very desirable I should 

 again draw attention, and that is a solution of biniodide of mercury 

 and iodide of potassium in distilled water. This is very easily pre- 

 pared by adding the two salts to the water until each shall be in 

 excess ; when this point of saturation has been reached the liquid 

 will be found to have a refractive index of 1 • 68, by far the highest 

 aqueous solution known to me. With this fluid there is no 

 difficulty or danger (apart from its poisonous nature) whatever, 

 either in mounting or preparing. Its advantages from an optical 

 point of view are considerable, and it may be used of any strength : 

 commencing with pure water, with a refractive index of 1 • 33, we 

 can go on progressively to 1 "465, which represents glycerine, still 

 on to 1*54 (Canada balsam), again onwards to 1*624, which 

 represents bisulphide of carbon, to 1 • 658 which represents the 

 monobromide of naphthaline, to 1 "662 the equivalent of a solution 

 of sulphur in bisulphide of carbon, until, undiluted, it finally 

 reaches its own maximum of 1 • 680 ; — thus we have the repre- 

 sentatives of all these media and an infinite number of others in 

 this one fluid. 



As mentioned at our last meeting, it is easily sealed with white 

 wax, and I have found the following a simple and effective plan of 

 doing so. 



The glass slip having been heated on the turntable, a wax cell 

 is formed by touching its surface with a piece of white wax ; in the 



