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II. Remarks suggested by Mr. Douglas's Account of a new Opto- 

 meter. By R. Templeton, Esq.* 



IN the Number of the Philosophical Magazine for May, 1869, a 

 notice appeared of an optometer, considered by the author an 

 improvement on that described by Dr. Donders. No exact mea- 

 surements being given, I am not quite certain that I exactly 

 realize the description given by Mr. Douglas ; but presuming 

 that I do, I think I can satisfy him that the indications of such 

 an instrument are fallacious. 



It is assumed, then, that a little screen (2 millims. in dia- 

 meter) is interposed betwixt the eye and a source of light 

 about 250 millims. distant from it : moving this source of light 

 nearer to, or further from the eye, a certain distance is deter- 

 mined, at which, as I understand Mr. Douglas, the two pencils 

 of light, one passing on either side of the screen, cease to be se- 

 parated from each other, one impression only being received 

 upon the retina. 



To test this, let two small screens be used, one the largest 

 possible that the pupil will admit of using, the other of smaller 

 dimensions ; and let exact measurements be made of their 

 breadth, and of the distances of the screens and of the source of 

 light, at the moment preceding the perception of the single 

 image. If now, to fix the ideas, a large horizontal section of the 

 eye be laid down upon paper, using the data given in Dr. Dan- 

 ders' s and other recent works, and these pencils be traced upon 

 it to their termination on the arch representing the first surface 

 of the retina, it will be found that they do not meet at that sur- 

 face. No reasonable hypothesis can make them do so. 



The case is simply an extreme example of the experiment de- 

 scribed half a century ago in Brewster's Journal : the looking at 

 a pin in a bright light through a series of minute holes (fifteen 

 to the centimetre) in a thin sheet of brass ; six or eight images 

 of the pin are seen, which are true images imprinted on the 

 retina, as can be easily shown. 



Next, let a leaf of paper be taken, on which is good-sized and 

 distinct printing, and let it be doubled up so that a line of let- 

 ters may be close to the edge of the fold : let this paper so folded 

 be brought to a shorter distance than that of clear vision, the 

 letters becoming misty and confused ; now move leisurely across 

 the paper, but much nearer to the eye, a somewhat dark-coloured 

 ruler, approaching it gradually to the folded edge : close in along 

 the edge of the ruler the letters will become distinctly visible 



* Communicated by the Author. 



