Salts and Saline Sub fiances. 15 
varied or directed juft as the Subject. re¬ 
quires. 
It may be alfo proper to take Notice, 
that no Kinds of Microfcope are fit for thefe 
Obfervations, but fuch as have an open 
Stage, whereon the Slips of Glafs with the 
Liquor upon them may be placed readily, 
and in a perfedft horizontal Pofition ; and 
moreover where they can be turned about 
freely, and without difordering the Fluid. 
Thofe Microfcopes where Sliders or Slips 
of Glafs, are thruft in between two Plates, 
as in Wilfon s, &c. be they in whatever Di- 
redtion, cannot therefore properly be em¬ 
ployed here.—The following Obfervations 
were all made by a double Microfcope, 
conftruded by Mr. Cufi\ in Fleet-fireet , 
with fome Regard to thefe Experiments. 
Its Figure refembles what is called com¬ 
monly the Double RefieBing Microfcope , and 
like that it receives Light from a Speculum 
underneath: but befides an Improvement 
in the Pofition of the Glades, its Stage is 
difencumbered with Legs, and quite open 
for the Reception of Objeds without Trou¬ 
ble ; its Motion is not by Jerks, as in the 
Microfcope juft now mentioned, but re¬ 
gulated by a fine-threaded Skrew, whereby 
it is adjufted to the Eye with great Eafe and 
Exactnefs; and all that try it will I believe 
judge it not only the moft convenient Micro¬ 
fcope for this Purpofe, but like wife for gene- 
9 ral, 
