224 NUNNELEY, ON THE RETINA. 
do not think any such precise relation in number or position 
exists. 
I know of no more beautiful object under the microscope 
than the outer surface of the retina of a Cochin cock, guinea 
fowl, or turkey, and particularly of a turtle. Could a lady 
deck herself in jewels so brilhant and beautiful, she would 
esteem herself the gayest in a ball-room. I know of nothing 
except the corneal facets of a coleoptera to compare with it. 
In some birds the colours are much more intense than in 
others. I thought it possible the colour of the feathers 
might have some connexion with this, but it does not ap- 
pear to have; for instance, the yellow globules of the canary 
bird are not more numerous or intense than those in the 
sparrow; the ruby in the brilliant game cock than in the 
gray guinea fowl: nor does there appear to be any difference 
in two similar birds of different plumages, as in a white and a 
variegated fowl. However, I do not think, in aquatic birds, 
they are so brilhant m tint as in land-birds, nor is there 
altogether so much difference in the size of the two-coloured 
globules. But, of all creatures which I have examined, the 
coloured globules are the largest and most distinct in the 
green turtle. ‘The ruby are, as in birds as a whole, larger 
than the yellow, and are more uniform in colour; whereas 
the tint of the yellow, like their size, varies very much, from 
a full canary, to nearly or completely colourless white. 
Similar, but not nearly so perfect globules, are found in 
some other reptiles. I have seen them in the toad and the 
frog; in the latter of which they are most distinct; and in 
some fish, the eel for mstance, globules of the same general 
size and appearance, but of a brown colour, more like those 
of the choroid, but larger, are to be found. So in some mam- 
malia brown globules of the same size and general characters 
are to be sometimes sparingly met with. In viewing these 
coloured cells they often appear as containing-a circular 
nucleus, which is, however, only an optical effect, for by 
adjusting the focus it always disappears. Hannover describes 
this appearance as resulting from the cells bemg conoidal, 
and both ends being seen at the same time. In some few 
eases I have thought the detached cells conoidal, but this is 
so very rare compared with the frequency in which no doubt 
can exist of the generally perfectly globular character of 
both ruby and yellow cells, that I rather incline to attri- 
bute it to the end of the rod in which the cell is partially 
imbedded being seen, than to the cause mentioned by 
Hannover. In one bird, the common fowl I believe, the 
globules had a short projecting spur, which was received into 
