 meedful to preferve the material Syftem. - 217 
— "To different fyftems, according to their fituations, the ex- 
paníe may exhibit very different phenomena. Although to our 
fyftem, or to us on this planet, it exhibits the blue concave of 
an all-fürrounding orb ; which, in the Milky Way, and in 
fome other parts of it, fhines with a brighter light, it may to 
other fyftems appear of other colours ; and exhibit to fome of 
them in fucceffion, according to their fituations, the feveral pri- 
mitive colours in the order, in which the rays of thofe colours 
are feparated and clafied. | 
Of one of thefe exhibitions, that of the blue colour, we 
have ocular demonftration. But why fhould the expanfe ap- 
pear to us blue, rather than green, or -of any other primitive 
colour ? If that appearance can be explained by the refrangi- 
bility of light, or by the feparation of it into its feveral colours, 
as perhaps it can, the other appearances of the. expanfe to other 
fyftems, naturally, if not neceflarily, follow. 
Experiments prove, that light is compounded of. diferendly 
coloured rays ; and that after it has paft through different -me- 
diums properly difpofed, the rays are refracted, or feparated and 
claffed, according to their different refrangibility ; and thew 
thofe colours in the order juft mentioned : that the three moft 
refrangible of them, the blue, the indigo, and violet, which 
poflefs one half of the fpace fpread over by the whole, are fo 
nearly allied in colour, that the laft, when confiderably fpread, 
are fcarcely to be diftinguifhed from the neighbouring blue : 
for which reafon, thofe three clafles appear.as one, at a great 
diftance from the refracting medium : and the blue thus cir- 
cumftanced, and uniting thole claffes, may therefore be faid to 
poftets : a {pace equal to the {pace occupied by all the reft. Bee. 
fm any fegment of a hollow fphere, fuch, fori asa 
Dd concave 
