198 Sir David Brewster on the Anatomical and Optical 
Although the parallel sides of the teeth do not form con- 
tinuous lines, yet they produce colours by interference in the 
same manner as if they were continuous; and it is by their 
influence that the secondary prismatic images are produced 
in a line perpendicular to the sides of the teeth. In the living 
as well as in the recent lens the faces of the teeth and of the 
fibres are all in optical contact, and light passes through them 
in every direction in the same manner as if the whole lens 
were a continuous solid. 
The toothed structure which has now been described, I 
have found in the salmon, haddock, herring, shark, and, in- 
deed, in the lens of every fish where I have looked for it; and 
in this class of animals it is generally very distinct, and almost 
always capable of producing the secondary prismatic images. 
In the salmon, however, the teeth are much narrower than in 
the cod, and the colours which they produce are less distinctly 
seen. 
Having thus determined the form and magnitude of the 
fibres and their teeth in a given lamina, it becomes interesting 
to ascertain if they suffer any change in shape or size at differ- 
ent distances from the centre of the lens. With this view I 
continued to remove one coat of the lens after another, till I re- 
duced it to such a minute nucleus that I could no longer use 
it. I then dried it at the fire, and having crushed it, I ob- 
tained small portions of lamina extremely near the centre: in 
this way I found that the fibres gradually diminished towards 
the centre of the lens, and the teeth in the same proportion, 
so that the number of fibres in any spherical coat or lamina 
was the same from whatever part of the lens it was detached. 
In the lens of a cod, I found that there were 2000 fibres in 
an inch at the equator of a spherical coat or lamina, whose 
radius was ths of an inch; consequently there must have 
been 2500 in the spherical surface*. If we now suppose that 
the breadth of each fibre is five times its thickness, and that 
each tooth is equal to the thickness of the fibre, or that five 
teeth are equal in breadth to a fibre, we shal! obtain the fol- 
lowing results for the lens of a cod four tenths of an inch in 
diameter : 
Number of fibres in each Jamina or oe 2,500 
WICH COAL cet vice’ sfechwcioas'eiss 
= hoot am,.eaeh: Dbre: \duios<sas ews 12,500 
—__—- - ——— in each spherical coat... 31,250,000 
— fibres in the lens ..........00008 5,000,000 
— teeth in the lens ...........00.. 62,500,000,000 
or, to express the result in words, the Jens of a small cod con- 
* The radius being .*, or *2, we have 3:1416 x°4 x 2000 = 2513. 
= 
