NA TURE 



[Dec. 25, 1884 



spar into two polarised beams perpendicular to each other 

 showed, after passing through a quartz plate, when viewed 

 through a Nicol prism, two halves of different colours in the 

 field of vision of the ocular lens, the colours of these halves 

 varying with the position of the Nicol prism. In the case, 

 however, of a so-called colour-blind or Dichromatic eye there 

 was a position of the Nicol prism in which the tw > different 

 colours in the halves of the field of vision appeared alike. The 

 position in which the colours appeared alike was not always the 

 same, but varied with the thickness of the quartz plate, with 

 the intensity of the light examined, and with the individuality 

 of the Dichromatic eye. An instrument of this description was 

 therefore an apparatus that might be depended on for the detec- 

 tion of colour-blindness. Normal eyes, in whatever position 

 the Nicol prism might be held, saw different colours ; cjlour- 

 blind eyes, when the Nicol prism was held in a certain position, 

 saw similar colours. The person whose eyes were to be tested 

 was made t> look through the apparatus towards one or other 

 source of light, and if at last, after a greater or less number of 

 turns of the Nieol prism, he saw but one colour, then that 

 person was proved to be colour-blind. The examination of a large 

 number of persons — about fifty — who confounded red and green 

 colours, usually distinguished as red- or green-blind, resulted in 

 showing that, with an equal intensity of light, and with the 

 same apparatus, a part of the colour-blind, on the Nicol prism 

 being placed in a certain position, saw similar colours, while the 

 remainder observed a corresponding similarity of colours with 

 another position of the Nicol prism, this second position pro- 

 ducing the same result for all persons of that category. The 

 colour blind being by this means separated into two sharply- 

 defined groups, neither shading into each other nor into the 

 group of normal eye-, it was a matter of much interest to in- 

 vestigate whether these two groups of colour-blind eyes showed 

 any other characteristic peculiarities. Among the methods 

 adopted for determining the colour-blind, that of examining 

 their spectrum, which was said to appear always shortened in 

 different ways in the different cases of the red- and green-blind, 

 was largely applied. It was seen, however, that there were so 

 many particulars to be taken into account as affecting the extent of 

 the spectrum, even in the case of sound eyes, that that method was 

 by no means available for the precise and distinctive measurements 

 here required. Another method, that of determining the neutral 

 point, seemed well adapted for exact physical utilisation. As 

 was known, Young's theory of the perception of colours as umed 

 that in the retina there were three different nerve-elements per- 

 ceptive of colour : one perceiving red, another green, and the 

 third violet, if the perceptive capacities of each of the nerve- 

 elements were traced as ordinates on the spectrum as abscissae, 

 three curves would appear, of which the first would have its 

 maximum in red, the second in green, and the third in violet. 

 If all three nerve-elements were acted on with equal force by a 

 stimulus of light, then the sensation of white was pr dm 

 but if they were affected in different degrees, then the sensation 

 of partial colour was the result. In the case of the colour-blind 

 there was, according to this theory, one curve wanting, the red, 

 or green, or violet. The two remaining ones, in that case, 

 must now cut each other in one point, and at the spot where 

 this point hit the spectrum, the colour-blind person, his colour- 

 perceptive elements thereby being affected with equal force, must 

 see white: at that spot was the neutral point. The finding of 

 this neutral point by means of a movable slil tin >ugh which a 

 spectrum could be viewed was, however, attended with several 

 inconveniences prejudicial to precision. Dr. Konig had there- 

 fore constructed a aratus for ascertaining the neutral 

 point. [This was described at length by the speaker in his 

 communication- to the Physical Society, Nature, vol. xxix. pp. 

 168, 496.] In nine persons, some of whom were red, and others 

 green-blind, the neutral points were determined, and the fol- 

 lowing observations made : (1) The neutral point was able to be 

 measured with great precision in each case of colour-blindness, 

 the average error being at the greatest, o'4 millionth of a milli- 

 metre, wave-length, and in the least o'l millionth of a milli- 

 metre. (2) The neutral point in the case of all col 

 persons, green- as well as red-blind, was situated at one spot of 

 the spectrum, in the green blue, between the wave-lengths 402 

 and 505 millionths of a millimetre. A division of the colour- 

 Mind into two groups, such as could be so exactly carried out 

 with the l< is, however, not possible through deter- 

 minations of the neutral point, for on the leucoscope colour- 

 blind pei sons of different descriptions had their neutral points 



quite close to each other, while eyes leucoscopically alike had their 

 neutral points most remote from each other. (3) With increasing 

 intensity of light the neutral point was displaced in all cases of 

 colour-blindness towards the more refrangible end of the spectrum. 

 Among the resultsof the measurements referred to, that cited under 

 (2) was of extreme interest for the theory of colour-blindness. 

 One conclusion it yielded was that the idea of the nature of 

 oil' im-hlindness derived from Young's theory received no sup- 

 port from the experimental examination of the consequences 

 deduced from it. Dr. Konig had occasion quite recently to 

 examine a so-called violet-blind person, and another who was 

 totally colour-blind, but he had not yet had time to reduce 

 the measurements he had carried out respecting these two 

 cass, and would therefore reserve further particulars of them 

 to another opportunity. The fact established in (1), that with 

 the apparatus constructed for ascertaining the neutral point 

 separate small sections of the spectrum may be so sharply 

 marked off and determined according to their undulatory length, 

 induced Dr. Konig to make use of this apparatus for investiga- 

 tions respecting the colour-perceiving capacities of normal eyes. 

 In co-operation with Dr. Dietrici he had first examined the 

 degree of sensitiveness to distinctions of colour in the different 

 partsof the spectrum between 650 and 430 millionths of a milli- 

 metre, undulatory length, and gave a summary of the results 

 thereby obtained which he ha.! already communicated to the 

 Physical Society (NATURE, xxix. 496, xxx. 308). It deserved 

 here, however, to be brought prominently forward that the 

 maximum of colour-sensibility of the two normal eyes coincided 

 with that spot of the spectrum at which the neutral point 

 occurred in the colour-blind, and that this maximum of colour- 

 sensibility shifted, in the same way as the neutral point, with 

 increase of the intensity of light towards the blue end. The 

 further investigations contemplated by Dr. Konig relate to the 

 colour sensibility beyond the wave-lengths 650 and 430 mil- 

 lionths of a millimetre, and determinations in regard to colour- 

 contra -.t. 



CONTENTS Page 



The "Challenger" Reports 165 



Geodesy and Measures of Precision 165 



Our Book Shelf:— 



McClellan's " Higher Teaching of Agriculture " . . 167 



Paul's " Text of Euclid's Geometry " 168 



Strasburger's " Kleine botanische Practicum fur 



Anfanger " 16S 



Letters to the Editor : — 



Dr. Koch and the Comma-Bacterium. — Prof. E. Ray 



Lankester, F.R.S. (Illustrated) 168 



On the Distribution of Honey-Glands in Pitchered 



Insectivorous Plants. — J. M. Macfarlane ... 171 

 Earthquakes in England, and their Study. — William 



White 172 



I he 'Cacao-Bug of Ceylon. — Dr. Henry Trimen . 172 



The .1/ ■ \thematics. — Angelus .... 172 

 The Pronunciation of Chinese Name=. — F. Porter 



Smith 173 



Explorations in Iceland, III. By Th. Thoroddsen 173 

 American Summer Zoological Stations. By Ralph 



S. Tarr • 174 



On a New Method for the Teaching of Science in 

 Public Elementary Schools. By W.Jerome Har- 

 rison. (Illustrated) 175 



Notes 179 



Our Astronomical Column : — 



Occultations of Aldebaran 1S1 



Encke's Comet 182 



Geographical Notes 1S2 



Roots. By H. Marshall Ward, M.A 183 



Focal Lines. By W. N. Shaw 185 



University and Educational Intelligence 186 



Societies and Academies 186 



