of different Species of Glass. 295 
Refracting Media. 
Nra'— On' 
Mn'— Nn' 
Ln'— Mb' 
Kn'—hn' 
In'— Kn 
N n — On 
Mra — Nn. 
L n — Mn 
Kn—Ln 
In — Kra 
Flint-glass, No. 139. 
Crown-glass, No. 9. 
1 
[l,93 
-2,00 
2,01 
2,07 
2,17 
Flint-glass, No. 139. 
Water, 
1 
[2,87 
3,01 
3,10 
3,33 
3,42 
Crown-glass, No. 93. 
Water, 
1 
[l,49 
1,51 
■1,55 
1,01 
1,58 
From these results, it is obvious that there are great anoma- 
lies in the ratio of the dispersion of the differently coloured rays 
in some refracting media. 
These experiments led me to make some observations on the 
influence of heat upon the refraction of fluids. By the least 
change of temperature, the refraction of all fluids becomes 
stronger in the lower part of the prismatic vessel, than it is in 
the upper part ; and hence every fluid acquires a kind of undu- 
lation, which prevents the coloured points of the spectrum from 
being precisely distinguished. In making these experiments 
during the night, when the temperature continually changes, I 
was obliged to stir the fluid every five or ten minutes, in order 
to render it homogeneous. These differences are not great in 
water, but in other fluids they are so considerable, that all 
the spectrum is dispersed and confounded, even if the vessel is 
shut up from the air. Hence it follows that we ought not to 
expect good object-glasses by substituting fluids in place of 
flint-glass *. We see also from these experiments, how difficult 
it must be to melt flint and crown glass of a perfect homo- 
geneity, since in every furnace of a glass-house the heat of the 
upper part of the crucible is almost one-third stronger than that 
of the lower part. 
In order to obtain the indices of refraction of the differently- 
coloured rays with more exactness, and in order to determine if 
the action which refracting substances exert upon the light of 
the sun is the same as upon artificial light, I adopted the follow- 
ing method. 
* This inference is surely unfounded ; for the small quantity of fluid in an ob« 
ject-glass will soon acquire an uniform temperature. Besides this, Dr Blair has 
actually made better object-glasses with fluid?, than ever were made with flint- 
glass.— Tea ns l. 
