BY MOLECULAR COALESCENCE. 189 
unnatural; and thus more likely to give false ideas than 
correct views of the structure of this organ. Of all the 
lenses which I have examined, I have met with none 
which answers the purpose so well as the lens of a young 
stickleback, of about three quarter of an inch or an inch 
long, but it should not be more. This part bemg of small 
size, admits of beimg seen entire without any further 
dissection than that necessary to remove it from the eye 
of the animal, and can be examined perfectly fresh, which 
in this case is indispensable; and as these animals are 
everywhere to be met with, the examination is within the 
reach of any one disposed to make it. Besides, I may 
observe, that if it be done with sufficient care, and not in 
an imperfect manner, as too many microscopical ex- 
aminations are, it exhibits some most interesting 
and beautiful physical facts. The following is ‘the 
manner in which the examination may be conducted. 
The head must be taken off whilst the animal is alive, 
and an eye removed without delay, which, being torn 
open by a sharp scapel, the lens enclosed in its cap- 
sule will escape, and is to be put immediately into a 
shallow cell filled with water, then a thin glass cover being 
placed over it, it must be examined at once. A half-inch 
lens at first, and afterwards a quarter-inch, are the 
best magnifying powers for the purpose, and it will, of 
course require to be seen by transmitted light. I may 
observe that the lens of this fish is nearly of a spherical 
figure, and that it is divided into an equator and two poles, 
the plane of the former being at right angles with that of 
the latter. The superficial layer of fibres, those especially 
requiring notice, and proceeding from each side of the 
equator towards the poles, may, from this circumstance, 
be divided into an anterior and a posterior set, each 
