the Index of Refraction of the Lens. 431 



in form or in comparative dimensions, the lens nearly globular 

 in each, yet the retina" is excessively close to the lens in each, 

 twice as close as it is in the cod's eye, and nearly three times as 

 close as it is in the sturgeon's ; is, then, the value of jju so great, 

 globular though the lenses be, as would be required should the 

 rays come to a focus on these little- distant retinas ? 



21. Testudo Mydas and the crocodile (Crocodilus sclerops) have 

 the cornea and lens closely approximating in form ; however, the 

 retina in the former is twice as distant, and its radius of curva- 

 ture twice as great. 



22. These comparisons could easily be multiplied, and most 

 singular differences shown to exist in even closely allied families. 

 Itjis quite possible to imagine that the conformation of the ani- 

 mal and the mode of life allotted to it may demand very essen- 

 tial differences in the sizes and proportional distances of the 

 various parts of the eye ; still that the refractive densities of the 

 humours should be subjected to the marvellous diversity required 

 to account for differences of comparative distance from lens to 

 retina of from 19 to 3, and of lengths of radii of curvature from 

 21 to little more than one-third that quantity, is not exactly what 

 would be expected. 



23. Would it be possible to make the eye tell its own tale ? 

 There being little difference in the refractive indices of water and 

 of ice, it might possibly not vitiate the results if freezing were 

 had recourse to; at any rate introduced error could be deter- 

 mined and allowed for ; and the experiment, if successfully tried 

 in regard to the human eye, the ox-eye, and the cod's, would 

 enable us to escape many an error likely to arise from a sole 

 consideration of the phenomena of vision as it occurs in man. 



The accompanying Plate, illustrative of the forms of the re- 

 fracting surfaces, &c, in the eyes of animals, and the following 

 Table are extracted from the work of Detmar Wilhelm Soem- 

 mering de Oculorum Hominis Animaliumque Sectione horizontali 

 Commentatio. 



EXPLANATION OF PLATE VI. 



Fine concentric lines were traced on a plate o n glass at distances of 1 mil- 

 limetre from each other, beginning with 3 millims. radius and ending 

 with 35. The plate of glass was then placed over the figures in the 

 younger Scemmering's work, and the curvatures, when accurately ad- 

 justed, read off. The distances of the surfaces were likewise read off. 

 The numbers so read off were next reduced to a common relation of 10 

 millims., from the first refracting surface to the last, for the purpose of 

 comparison, and are in this state figured in the Plate. 



