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the density of these humours and that of wa- 

 ter, to do the same by rays transmitted by this 

 fluid, so that such rays are not brought to a 

 focus sufficiently soon. Hence, divers in some 

 places, are in the habit, when they descend into 

 the water, of using extremely convex glasses, in 

 shape almost like the lens of fishes, and turning 

 their eyes by this means, as it were, into those 

 of an aquatic animal. But how do reptiles ma- 

 nage this ? Not by using spectacles, nor by in- 

 creasing the density of their humours ; but by 

 increasing the distance between the cornea and 

 retina which they effect by compressing the 

 globe of the eye by proper muscles given to them 

 for that purpose so that the rays which, from 

 the defective refracting powers of their humours, 

 would have otherwise formed a focus beyond the 

 retina, now form a focus upon it. When again 

 in the air they relax these muscles, and the reti- 

 na again approaching the cornea, still receives 

 the focus of the rays, which, as passing now 

 through air, are sufficiently refracted for the pur- 

 pose. Whether we regard then the heart and 

 blood-vessels, the respiratory organs, or those of 

 the senses, in these tribes, we trace equally dis- 

 tinctly the main object which nature had in view 

 in their construction. The motions of the iris 

 in reptiles now for the first time perceptible 

 are still extremely languid, and the form of the 

 pupil is very various, being rhomb-shaped in the 

 frog, vertically oval in the crocodile, &c., but this 

 probably makes no difference in the phenomena 



