156 PROCTOR, ON LIGHT. 
aggregated. The white lead is somewhat yellowish, evidently 
transparent, but not so clear as the magnesia. 
By reflected light the magnesia is comparable to a fine 
sample of crystalline moist sugar, and the lead has an evident 
lustre and transparency, though in a less marked degree than 
the magnesia. 
In those specimens mounted with balsam the magnesia is 
seen to be very transparent, consisting principally of roundish 
bodies, which have a dark centre when beyond focus and a 
bright centre when within focus, showing that the magnesia 
has less refracting power than the balsam. The white lead is 
very evidently transparent, and has bright centres when be- 
yond focus, which are diffused when it is brought within the 
accurate focus, showing that the lead has greater refracting 
power than the balsam. 
I have here two specimens of white lead, to illustrate the 
difference between that made by the usual troublesome pro- 
cess and that made by some of the more speedy processes 
which have at various times been tried, but have always fallen 
out of use again, because the precipitated lead was deficient in 
body, in consequence of its consisting of larger particles, as 
you will perceive. In commercial white lead the larger par- 
ticles are aggregations of smaller ones, consequently look 
white and opaque, but in the precipitated variety they are 
solid crystals, clear and transparent. 
I fear you will think I have occupied too much time with 
a matter more interesting to painters than to microscopists ; 
but this is not the case, it is a subject of general optical im- 
terest, and specially connected with the mounting of micro- 
scopical objects. Why are sections of wood mounted dry if 
to be viewed as opaque objects, and in balsam if to be exa- 
mined by transmitted light? What I have already said has 
answered the question. Why are starches best seen when 
mounted in gelatinous media? When examined dry, the 
great refraction the light undergoes in passing through them 
makes them appear black, except one small focus of light ; if 
mounted in balsam (the refracting power of which so nearly 
concides with their own), their minute structure becomes 
invisible from the exceeding transparency imparted, accord- 
ing to the rule already pointed out. To overcome the lens- 
like power of the granules, without rendering their super- 
ficial markings invisible, we require to select a medium of 
refracting power between that of air and of balsam, and this 
we find in gelatine, &c. 
I have here three mountings of tous-les-mois, in illustra- 
tion of this fact. You will observe that which is in balsam 
