Remarks on a New Optometer. 341 



merely indicated a principle on which an instrument might be 

 constructed for accurately defining the region of accommodation. 

 I did this after innumerable experiments had convinced me of 

 the existence of the phenomena described, and careful considera- 

 tion had led me to the explanation of the same. After I had 

 arrived at my results I referred to Brewster's Journal ; I found 

 there the experiment of Le Cat, to which, I presume, Mr. Tem- 

 pleton refers when he states, "the case is simply an extreme 

 example of the experiment described half a century ago in Brew- 

 ster's Journal." In my former paper I have referred to Brewster's 

 Journal (vol. iv. p. 89) ; but the experiment there described is 

 not that described by me, as comparison will prove ; it is not 

 applied to any useful purpose ; and, as I have already insisted, 

 the explanation given is not the true one*. 



The first production of multiple images by two or more orifices 

 is due to Father Schemer (1685); the application of Father Schei- 

 ner's experiment to optometry to Porterfield (1759) f- Helmholtz 

 recommends as a means of finding the far point the use of a point 

 of light, a small opaque body being passed close to the eye, and 

 the light moved to or from the eye until the opaque body is no 

 longer seen in transit. I have suggested the use of two or more 

 luminous points; and instead of moving these luminous points, I 

 proposed to move the opaque body to or from the eye until it is 

 seen single. The proposed method has several advantages : it is 

 easily applied, more strictly defines the distance required, parti- 

 cularly in the case of an inexperienced observer ; for the several 

 shadows are very distinct, and their coalescence is therefore easily 

 traced, easier than the gradual disappearance in Helmholtz's ex- 

 periments ; and the multiplication of shadows may be applied 

 to purposes other than optometrical ; for example, if several 

 stars appear to be one only, in consequence of the encroachment 

 on each other of their circles of confusion, by passing a small 

 object close to the cornea, the number of shadows seen will indi- 

 cate the number of luminous points. 



Mr. Templeton has referred to hypotheses of lens-adjustment, 

 the most difficult portion of physiological optics. Rather than 

 repeat what has been written elsewhere, I would refer Mr. Tem- 

 pleton to the chapters treating of this subject and the history of 

 its development in Professor Helmholtz's work referred to 

 already. The treatment is very full, and the several theories 

 proposed are tested by experiment. Mr. Templeton's experi- 

 ment with the ruler may be explained on the same principle as 

 the formation of images by a pinhole, the frowning of myopes, &c. 



Mr. Templeton states, " physicists have assumed, on quite in- 



* Another refutation of the explanation referred to appeared ia the Phi- 

 losophical Magazine for June last, in a paper hy Mr. J. L. Tupper. 

 t Helmholtz, Physiologische Optik. Leipzig, 186/. 



