168 Transactions of the Society. 



centre of the circle thus formed, when cold, a drop of the solution 

 is placed, and on this the thin glass cover. 



The cover can be fixed by heating an ordinary gun-punch (or 

 other metalhc ring) to the melting-point of wax, and placing the 

 cutting edge on its upper surface ; the weight of the punch as the 

 wax melts soon adjusts the cover in its place, and when cold the 

 excluded solution is cleaned off. 



Two or three coatings of gold size and one of shellac finally fix 

 it, as in the case of the phosphorus. 



This fluid is so dense, its specific gravity being 3*02 (as kindly 

 determined for me by Mr. C. Gr. Stewart, of St. Thomas's Hospital), 

 that almost any microscopic object will float on its surface ; this is 

 the case with diatoms, for example, and consequently any which 

 may become detached will still be found in contact with the cover, 

 and may thus possibly present themselves under different aspects. 



Its refractive index being 1 • 68, the visibility of diatoms, when 

 mounted in it, is represented by the number 25 as compared with 

 11 in Canada balsam — in other words the image is nearly 2^ times 

 as strong; this is no doubt very inferior to that yielded by 

 phosphorus, in which the strength of the image is 6 times as great 

 as in balsam, but nevertheless, Am/phijileura pellucid a is very 

 easily resolved in it, and on looking over a slide, mounted last 

 evening, not one valve was found (and they were delicately marked), 

 which was not resolved without any trouble under Zeiss' homo- 

 geneous |. 



For muscular fibre, on the other hand, a strong solution is not 

 suitable, since the high refractive power of the object approaches that 

 of the medium, and the resulting image is consequently very faint, 

 but as every other medium of a lower index than 1 • 68 can, by 

 dilution, be represented by it, any degree of visibility down to that 

 yielded by water can be obtained. 



For marine animals a weak solution is probably well adapted, as 

 about a 1 per cent, solution (5 minims to the ounce) will give the 

 specific gravity of sea-water, with no appreciable difference in the 

 refractive index ; and the same strength appears suitable for some 

 vegetables. How far the colours of these may fade can only be 

 determined by time, but a limited experience shows that the colour- 

 ing matter of the petals of flowers is dissolved out, although the 

 action on chlorophyll appears in some cases to be small, after two 

 or three weeks' exposure. 



Although the dispersive power of a mounting medium is not of 

 importance, it may be mentioned as a matter of interest, that the 

 dispersive power of this fluid is excessively great, being equal to 

 • 05483 (that of very dense flint glass, n = !• 802, being only 

 0" 03287), and the extension of the blue in comparison with the 

 red, much greater than that of any other known substance, as I am 



J 



