56 Dr. W. Pole on Colour -Blindness. 



retina 3 and alludes to the analogy between these and colour- 

 blindness. 



Helmholtz thinks it right to devote considerable space 

 (pages 376 to 382) to the discussion of the theory of Hering. 

 He considers it a modification of Young's theory, giving a 

 long description of it, illustrated by mathematical formulae, 

 and he says it will explain the facts of colour combinations as 

 well as, but not better than, Young's theory. Then follows 

 much controversial matter, founded on arguments and state- 

 ments by Hering, but very little of which affects the question 

 of colour-blindness. 



The most important novelty in HelmhohVs second edition 

 is contained in section 21, on The Intensity of the Sensation of 

 Light. This section is a long one, extending over nearly 100 

 pages ; and a large portion of it, i. e. from pages 401 to 473, 

 is entirely new. It contains a most elaborate essay on 

 " Helligkeit," a word for which we have no perfect equi- 

 valent, but which is generally rendered by "brightness" 

 or "luminosity.'" The matter is principally devoted to the 

 relations between " Helligkeit " and colour, which appear to 

 be exceedingly complicated and difficult ; so much so, indeed, 

 that Helmholtz, with the modesty that so often accompanies 

 great knowledge, expresses distrust of his power to- deal 

 satisfactorily with them. 



Fortunately, however, we have no occasion to speak of these 

 relations, further than as they are connected with an entirely 

 new determination of the three fundamental colour- sensations , 

 by a method in which " Helligkeit " (which for this purpose 

 I translate as "luminosity") takes a part. This investigation 

 may be said to begin on page 444, with a dissertation, "'On 

 the relations between Sensitiveness to Colour and Sensitive- 

 ness to Luminosity/' The author raises the question how far 

 observations on the spectral colours, having these elements in 

 view, will bear on the nature of the three physiological colour- 

 sensations, and having satisfied himself on this point he 

 undertakes the investigation. 



I can only give (and it would not be right to do more) the 

 barest outline of this interesting but difficult and complicated 

 problem.- It adopts in the first place the view of the funda- 

 mental sensations applied shortly before by Konig and Die- 

 terici in attempting a similar inquiry. They had analysed 

 the spectral hues by a great mass of experiments, using three 

 " Elemental colours" E, G, V, taken from the two ends 

 and the middle of the spectrum, by combinations of which 

 every other spectral hue could be produced. These, on the 



