160 
422. But though the Newtonian hypothesis ap- 
pears thus to fail in its general application to the per- 
manent colours of natural bodies, yet, in the exam- 
ples of liquors which possess an uniform composition, 
and which, at different thicknesses, exhibit different 
colours, when viewed by transmitted light, it may, 
perhaps, to a certain extent, be deemed satisfactory ; 
for as all bodies intercept a portion of the light that 
falls on them, it is reasonable to suppose, that the in- 
creasing mass or density of the fluid should furnish 
an increasing obstacle to the passage of light, until, 
at the thickest part of the liquor, the least refrangi- 
ble, or red-making rays alone are transmitted ; and 
such seems to have been truly the cause of the red- 
ness which the upper portion of the liquor presented 
in Newton's experiment (419.), and which Dr Hal- 
ley's hand exhibited at great depths in the sea *. 
The red colour of the sun too, in certain states of the 
atmosphere, seems to arise from a similar extinction 
of the more refrangible rays ; and so, likewise, the 
light from our lamps, in very hazy weather, is not 
only diminished in intensity, but varied in species, so 
as to approach nearly to perfect redness. 
423. But when vegetable infusions are changed in 
colour by acids and alkalis, a great discordance seems 
to obtain between the imputed properties of those 
substances, and the established qualities of the seve- 
ral rays of light. For if acids do dissolve and atte- 
nuate (419.), they must, on the mechanical hypothe- 
sis, be considered to facilitate the passage of light ; 
and yet, after the addition of an acid, the more re- 
Optics B. i. part 2. prop. 10. 
