i 3 4 HERMANN VON HELMHOLTZ 



saturated blue colour of the ultra-violet rays was absolutely 

 different from the almost totally white hue of the light dis- 

 persed by the dead retina. The wave-length of ultra-violet 

 light was also measured by Esselbach in Helmholtz's laboratory 

 at Konigsberg, and under his supervision. This research was 

 communicated by Magnus to the Berlin Academy in December, 

 1855, under the title, ' Measurement of the Wave-length of 

 Ultra-violet Light, by E. Esselbach,' with an appendix by 

 Helmholtzon the physiological-optical results of these measure- 

 ments. He gives an extended comparison of the relations of 

 the length of light-waves with that of musical intervals, accord- 

 ing to which the entire visible portion of the solar spectrum 

 comprises an octave and a fourth ; his tables show how little 

 analogy there is between sensations of tone and of colour, 

 since the whole of the intermediate degrees between yellow 

 and green are compressed into the breadth of a small semi- 

 tone, while at the end of the spectrum there are intervals as 

 large as a major or minor third, in which the eye is unable to 

 perceive any alteration of colour-tone. 



His great treatise on Accommodation was now approaching 

 its conclusion. It was published in 1855, in Graefe's Archiv 

 f. Ophthalmologie, and contributed an extraordinary mass of 

 new points of view, methods, and results to physiological 

 optics. 



The priority of one fundamental discovery, as previously an- 

 nounced in the Monatsberichte of the Academy, had indeed to 

 be forgone in favour of Cramer, i. e. that the lens in the resting 

 condition of the eye, when it is accommodated for far vision, 

 is not in its natural form, but is flattened by the surrounding 

 structures, but that the pull of Briicke's muscle enables it 

 to resume its natural form of marked curvature and greater 

 thickness, in virtue of its elasticity results which he obtained 

 not by watching the alteration of form, or displacement of 

 the media of the eye in accommodation, but by investigating 

 the changes of the weak light-reflexes first observed by Sanson 

 within the pupil, which take place at both surfaces of the 

 crystalline lens, and are sufficient to account for accommo- 

 dation. But many difficult questions still remained, which 

 could only be solved by a gifted mathematical physicist. Such 

 were the exact determination of the inner and outer surfaces 



