10 Mr. R. Templeton's Remarks on Mr. D< 



aud sharply defined. Let 

 now a large capital letter 

 be pinned up against the 

 window-shutter frame (so 

 as to have theadvantageofa 

 good light), and bring (see 

 figure) the eye carefully up 

 to it to the distance of ordi- 

 nary correct vision ; then 



slip in between the eye and the capital letter the folded-up paper 

 with the ruler, so that the letters along its edge, distinctly seen, 

 may be observed in the line of the capital letter : the smaller 

 letters and the capital letter can now be seen at the self-same 

 moment with perfect distinctness. The result is very striking, 

 and, I need scarcely add, goes to the root of all hypotheses of 

 lens adjustment*. 



Physicists have too readily, and on quite insufficient evidence, 

 adopted the opinion that the eye is simply a camera, with a lens 

 to form a picture, a retinal surface to receive a picture, and an 

 adjusting-power for distance. Since the date of my paper, in 

 vol. xxxii. S. 4. of Phil. Mag. p. 425 f, I have seized every 



* I am acquainted with a case of reticular cataract in which three of 

 the meshes are sufficiently open to permit light to enter freely when at 

 night the pupil is dilated : three distinct images of distant lamps are then 

 plainly seen ; this is incompatible with the accommodation hypothesis. 

 Vide Phil. Mag. S. 4. vol. xxxii. p. 428, § 16. 



f In that paper I showed that if an opaque object be looked at atten- 

 tively in a good light so as to be perfectly well seen, D being the distance 

 of the object from the cornea, and S the diameter of the pupil, or of any 

 other diaphragm held quite close to the cornea which permits a conical 



pencil of rays to pass on into the eye, then is jj a constant quantity ; and 



the rule holds good up to a metre or two, beyond which absolute definition 



ceases. The angle answering to jj is about 20' in a fair light; but its 



value decreases as the degree of illumination increases, constant, however, 

 for the same light. 



In the paper above alluded to I have likewise shown that, as far as can be 

 at present surmised, the impulses are received in each case on a portion of 

 the retina of which the diameter is exactly equal to the depth of the sentient 

 layer of that part. 



The appearance of printed matter seen through a narrow slit (0*25 mil- 

 lim. in breadth) is very curious, and merits further examination : close along 

 the edges of the slit, above and below, the lines of letters are distinctly 

 seen, the central portion a heap of short lines in singular disorder. 



Lastly, I may add that I have been able many times to make out indi- 

 stinct objects by reducing the size of the diaphragm supplied by the opti- 

 cian; and I think Sir John Herschel makes a remark which comes to much 

 the same thing, if I remember right, in his ' Cape Observations.' 



