BY MOLECULAR COALESCENCE. 23 
particles,—an attraction which is as their distance from 
the centre. So that, this force being least at the centre, 
and greatest at the surface, these differently shaped parts 
will possess a density varying according to their distance 
from the centre and with it also will vary their refractive 
power. The effect of the diminution of refractive power 
being in these calculi as the distance from the surface is 
well seen on comparing them with the lenses of very small 
fishes. The latter, being made densest at the centre and 
least so at the surface, for the purpose of preventing 
spherical aberration, present, under the microscope, when 
examined by transmitted light, a spherical figure, and can 
be seen to possess the property of a true lens, by one part 
magnifying the other, so that the markings on the remote 
surface of such a lens always appear, when seen through its 
entire thickness, considerably larger than those on the 
near one, and if its transparency be not much impaired, 
they can be seen very distinctly. Whulst, on the contrary, 
an artificially formed calculus of about the same size, 
being exactly the reverse in respect to the relative density 
of its different parts, causes such an amount of spherical 
aberration that its form, though it may be perfectly 
spherical, appears under the microscope, when viewed by 
transmitted light, to be flat, and any marking on its sur- 
face furthest from the microscrope cannot be seen at all 
magnified by looking through it, as was observed in the 
crystalline lens of the fish. This subject will be further 
discussed when treating of the development of the crys- 
talline lens generally. But, even so far as it has been 
considered, it removes the difficulty which at first presented 
itself, and furnishes incidental evidence in favour of the 
principles and accuracy of the reasoning which have been 
all along resorted to. 
