170 
GllEENLAND VOYAGE. 
There was an anomaly in these optical phe¬ 
nomena, with regard to the land, which was, for a 
this order (such as the exhibition of one erect and one in¬ 
verted image of distant objects, under the influence of un¬ 
usual atmospheric refraction, &c.), which may, perhaps, be 
extended to the illustration of the more complicated pheno¬ 
mena. This explanation refers to tire peculiar refractive 
effects of media of various densities, when brought into con¬ 
tact, or united by mutual penetration ; and is satisfactorily 
and beautifully illustrated, by experiments with liquids of 
different specific gravities, and air of different densities. 
The first mode of illustration was accomplished, by putting 
a 9mall quantity of clear syrup into a square phial, and 
carefully pouring on the top of it an equal quantity of 
water ; small objects seen through the incorporated medi¬ 
um (namely, through that part of the phial where the syrup 
and the water had slightly combined), appeared inverted 
with an erect image above. A similar experiment with 
water and spirit of wine above it, gave similar results ; 
but when the object examined was placed nearer to the 
rarer medium (the spirit of wine), the inverted and erect 
images were seen below. Analogous effects were also ob¬ 
served, by looking along tire side of a red-hot poker, at a 
paper ten or twelve feet distant. (Philos. Trans, for J810.) 
Another ingenious illustration of these phenomena was 
contrived by Dr Brewster. His very satisfactory experiment 
« consists in holding a heated iron above a mass of water, 
bounded by parallel plates of glass. As the heat descends 
through the fluid, we have a regular variation of density, 
which gradually increases from the surface to the bottom. 
If we now withdraw the heated iron, arid substitute a cold 
body in its place, or even allow the air to act alone, the su- 
