7 * 
Dr. Young's Lecture 
conceiving their thickness to be immediately increased, and to 
produce an immediate elongation of the axis, and an increase 
of the central curvature ; while the lateral parts co-operate 
more or less, according to their distance from the centre, and 
in different individuals in somewhat different proportions. On 
this supposition, we have no longer any difficulty in attributing 
a power of change to the crystalline of fishes. M. Petit, in a 
great number of observations, uniformly found the lens of 
fishes more or less flattened : but, even if it were not, a slight 
extension of the lateral part of the superficial fibres would allow 
those softer coats to become thicker at each vertex, and to form 
the whole lens into a spheroid somewhat oblong ; and here, the 
lens being the only agent in refraction, a less alteration thqn in 
other animals would be sufficient. It is also worthy of inquiry, 
whether the state of contraction may not immediately add to 
the refractive power. According to the old experiment, by 
which Dr. Goddard attempted to show that muscles become 
more dense as they contract, such an effect might naturally 
be expected That experiment is, however, very indecisive, and 
the opinion is indeed generally exploded, but perhaps too has- 
tily ; and whoever shall ascertain the existence or non-existence 
of such a condensation, will render essential service to physio- 
logy in general. 
Dr. Pemberton, in the year 1719, first systematically dis- 
cussed the opinion of the muscularity of the crystalline lens. * 
He referred to Leeuwenhoek's microscopical observations; 
but he so overwhelmed his subject with intricate calculations, 
that few have attempted to develope it : and he grounded the 
* De Facilitate Oculi qua ad diversas Rerum distantias se accommodat. L. B. 1 7 19. 
Ap. Hall. Disp. Anat. IV. p. 301. 
