HERMANN VON HELMHOLTZ 



not omit to state that Helmholtz shortly afterwards 

 arrived independently at the same result. With a 

 noble modesty, for which I thank him, he declared 

 himself convinced, after an examination of the papers 

 sent to him, that the enigma of accommodation, upon 

 which so many enquirers had exhausted their in- 

 genuity, was in reality solved as to the main point, 

 and very little now remained for him to do in the 

 researches he contemplated.' Helmholtz, in the first 

 instance, contrived a little instrument, the phakoscope, 

 by which the images in the eye may be seen even 

 better than in a darkened room. Then he invented 

 an instrument for the purpose of measuring the size 

 of any image reflected from a curved surface. This, 

 one of the most ingenious devices, is known as the 

 ophthalmometer. It depends essentially on the displace- 

 ment to one side caused by holding a plate of thick 

 glass obliquely in front of any object. The object 

 may thus be displaced through a distance equal to its 

 breadth, and the angular movement may be read off 

 on a graduated disc attached to the axis rotating the 

 glass plate. Helmholtz used two plates, rotating in 

 opposite directions, so that the object was displaced 

 both to the right and left, and the image was viewed by 

 a short focus telescope placed behind the plates. The 

 instrument may be graduated empirically, or it may 

 be used with the aid of a formula by which the size 

 of the object may be readily calculated. An image 

 is first obtained by throwing into the eye reflec- 

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