HERMANN VON HELMHOLTZ. 713 



borderland of physics, pliysiology, and psychology he won a place that 

 is all his own This thorny region has been invaded by others from 

 both sides, bnt it is not too much to say that You Helmholtz, in his 

 triple niastery over anatomy, mathematics, and physics, had unique 

 qualilicatious for the task. 



To the oculist he gave the ophthalmoscope, and thus made it possi- 

 ble to investigate the conditions of the inmost recesses of the living 

 eye. If tlie eye be illuminated a portion of the light returns from the 

 hinder surface, is brought to a focus by the lenses of the eye itself, and 

 forms an image of the retina in external space. To see this was no 

 easy matter. If the patient's eye were focused on a luminous object 

 the image would coincide with the source of light, and, even if other- 

 wise visible, would be lost in the glare. If he looked elsewhere the 

 image would move, but, inasmuch as the lenses can not be adjusted to 

 the clear vision of any object nearer than about 10 inches, that is the 

 minimum distance from the eye at which it can form the image of its 

 own retina. To see this clearly an observer without appliances must 

 place himself at least 10 inches from the image, that is, at 20 inches 

 from the patient. At that distance the view would be so limited that 

 no result could be obtained. 



Yon Helmholtz, however, convinced himself that, if these difticulties 

 could be overcome, the image of a brightly illuminated retina could be 

 seen. He made the observations through a small hole in the center of 

 a mirror, which reflected light into the eye under examination. Then, 

 by means of a lens, he shifted the ijosition of the image backward 

 until the relative positions of the observer and the patient were such 

 that, according to calculation, the retina should be visible. 



Again and again he tried and failed, but he was convinced of the 

 validity of the theory, and at last the experiment succeeded. From 

 that time the occulist has been able to look into the darkness of the 

 pupil, and to see through the gloom the point of entry of the optic 

 nerve and the delicate network of blood vessels by which it is sur- 

 rounded. 



The great monograph on the Sensations of Tone appeared in 1803. 

 The theories advanced were novel, but, though some points are still 

 open to dispute, they have as a whole been generally accepted. The 

 aim of the work was ambitious, being nothing less than the discovery 

 of the physical basis of the sensations which affect us when listening 

 to consonant and dissonant musical intervals, respectively. The gen- 

 eral nature of the solution arrived at is now well known. If two notes, 

 which differ but little from unison, are produced together, throbbing 

 alternations in the intensity of the sound are heard as beats. If the 

 interval is gradually increased the beats become quicker, till at last 

 they can no longer be distinguished separately. According to Yon 

 Helmholtz, however, they produce the effect of dissonance. "The 

 nerves of hearing," he says, "feel these rapid beats as rough and 



