MEMORANDA. 257. 
former of these distances divided by twice the latter will 
give the tangent of half the angle of aperture. The angle 
corresponding to this tangent, therefore, being found by the 
aid of the ordinary trigonometrical tables, the double of it 
will be the angle required. 
Thus, with an excellent quarter by Ross, I find the dis- 
tances between the flames 214 inches, and the distance of 
the line joining them from the object-glass 123 inches. 
Hence, dividing 21:5 by 24°5, I obtain 8775, which I find 
to be the natural tangent of 41° 16’; consequently the angular 
aperture is 824 degrees.—P. Gray, 7, St. Paul’s Villas, 
Camden Town. 
Glycerine Jelly.—In most of the works on the microscope, 
gelatine is mentioned as a medium for the mounting of certain 
objects, and several mixtures of gelatine with other sub- 
stances have been suggested, but none of these have suc- 
ceeded so well in my hands as the following, which I am 
induced to publish, because all my friends to whom I have 
communicated it have tried and approved it. 
It is the only medium which will preserve the natural 
colour of vegetable substances. 
The beautiful green of some mosses, I mounted two years 
ago, is still as fresh as on the day they were gathered. 
My formula is as follows: 
Take any quantity of Nelson’s gelatine, and let it soak 
for two or three hours in cold water—pour off the super- 
fluous water, and heat the soaked gelatine until melted. To 
each fluid ounce of the gelatine add one drachm of alcohol, 
and mix well; then add a fluid drachm of the white of an 
egg. Mix well (whilst the gelatine is fluid, but cool). Now 
boil, ‘until the albumen coagulates, and the gelatine is quite 
clear. 
Filter, through fine flannel, and, to each fluid ounce of the 
clarified gelatine, add six fluid drachms of Price’s pure 
glycerine, and mix well. 
The objects intended to be mounted in this medium are 
best prepared by being immersed, for some time, in a mixture 
of glycerine, one part, and dilute alcohol (six of water to one 
of alcohol), one part. The bottle of glycerine jelly is put 
intoa cup of hot water, until liquefied, where it is used in the 
same way as Canada balsam, excepting that it does not require, 
and must not be subjected to, the same amount of heat. A 
ring of asphaltum varnish round the cover completes the 
mounting. 
As I have found lately, that in mounting Conferve and 
