152 On the Affinities of Bodies for Light ; 
of solids and liquids; they are disposed in the form of 
prisms, z.e. they are terminated by surfaces oblique from 
each other; and upon looking throngh them, we observe 
the deviation experienced by the rays of light, occasioned 
by their entrance and exit under a certain obliquity. 
But how can we make a prism of air? In the same man- 
ner as a liquid prism, by inclosing air in any cavity, termi- 
nated, at the points where the light enters and departs, by 
transparent plates of glass, the respective inclination of 
which, as well as the refracting angle, is well known. If 
the apparatus is so constructed that what is called a vacuum 
in physics may be made and maintained round the glass 
plates in which the air is contained, the atmospheric air may 
then be considered as a prism; because, the essential condi- 
tions being the difference of the mediums which the light 
ought to traverse, and the obliquity of incidence, these are 
obtained if-the hollow prism is left empty ; or if it be filled 
with an aériform substance different from the ambient air; 
or, lastly, with common air, but of a different density. 
Newton induced Hawksby to pursue some experiments of 
this kind; but Hawksby’s prism having only a very small 
refracting angle, and the means of observation of the visual 
angles being at that time very much below the degree of 
precisior LW hick may be obtained at present, these pursuits 
were left unfinished; and they could not have fallen inte 
better hands. 
At the suggestion of M. de la Place, the Institute charged 
the authors of the present memoir with the further develop- 
ment of the above pursuits; and in detailing their results 
they do becoming homage to the illustrious authors of the 
Static Chemistry and the Mecanique Céleste: «* The sub- 
ject of all our researches,” they say, ** was indicated in the 
above works, and the conversation and advice of their au- 
thors furnished us with the means of making our re- 
searches.’? 
Borda had undertaken the same researches with instru- 
ments, to the perfection of which he had ereatly contributed 3 
he died, however, before having terminated his labours, and 
we bibve only recovered some “slight traces of them, But 
his 
