598 



NA TURE 



[February 3, 19 10 



now, under the same conditions of seeing in which the 

 rings appear continuous — their real form — in the smaller 

 glass, the aperture be increased, the rings will be seen to 

 break up into detached masses. It is very rare that they 

 ever appear otherwise in a very large glass, for the seeing 

 is almost never good enough thus to show them, accuracy 

 of definition demanding much better air for a large than 

 for a small aperture. 



Now what happens to the lines of the image of a star — 

 for the rings are simply circular lines — must happen in 

 the same manner to planetary lines. They too must break 

 up into a mosaic in a large glass whenever the rings do. 

 This shows that planetary lines would necessarily assume 

 a mosaic in a large glass contrary to their true form of 

 continuous linearity. Percival Lowell. 



Lowell Observatory, Flagstaff, .Arizona. 



Colour. blindness. 



In the summary of the case of Mr. John Trattles pub- 

 lished in Nature of January 27 the expression is used : — 

 " Dr. Ettles, who had examined Mr. Trattles, and 

 was of opinion that he was not colour-blind, was also 

 present." 



That is only relatively accurate, and it is in the reserva- 

 tion that the real point lies. I examined Mr. Trattles on 

 May 10, II, and 12 last year. His spectrum range included 

 both B and H lines — actually, 0-75 /i in the red to o-3q p. 

 in the violet. His sense of colour was less good about 

 054 )i than in the rest of the spectrum. Indeed, it was 

 what one might call " bad." It was the presence of this 

 defect which caused him to give such contradictory rephes 

 at Sir Wm. Abney's examination. My certificate, which 

 was read in the House of Lords by Lord Muskerry in 

 the debate referred to, viz. June 30, 1909, expressly 

 stated that Trattles was competent to navigate a ship. It 

 did not state that his colour vision was in all respects 

 perfect. 



Sir Wm. Abney and those associated with him concluded 

 that, as the result of their tests, Trattles was " completely 

 red-blind." I, saw in those tests simply, a confirmation of 

 my Qwn . conclusions. We were at one in the symptoms, 

 but we strongly differ in the diagnosis. If the Board 

 of Trade starboard light were a yellow-green, Trattles 

 would be unfit ; but it is a blue-green, and he sees it 

 perfectly. 



As to red, he is anything but scoterythrous ; that is 

 what comes of being obsessed with a colour-vision .theory. 



One other point, the " recondite test of simultaneous 

 contrast colours " is very much open to criticism. .'\ 

 simultaneous colour contrast is not a "colour" in the 

 ordinary sense : it possesses no dominant wave-length ; it 

 is an optical illusion. Is an optical illusion so accepted 

 and clear a test that it may be used as a touchstone to 

 determine whether a man's career shall be ruined or not? 



William Ettles. 



34 Wimpole Street, Cavendish Square, W., 

 January 30. 



We regret that we were in error in stating that Dr. 

 Ettles was of opinion that Mr. Trattles was not colour- 

 blind. It is interesting to know that he found the colour 

 vision " less good " at a point in the spectrum not far 

 removed from the region where the so-called " red-blind " 

 person has a so-called " neutral band." The accurate 

 determination of the spectrum range and of the deviations 

 from the normal within that range constitutes the funda- 

 mental problem in discriminating colour-blindness. The 

 accuracy of the determination, however, depends upon the 

 methods adopted and the precision of the application of 

 these methods. 



With regard to the simultaneous contrast test, Dr. Ettles 

 states that it is an optical illusion. If this be admitted, it 

 is constant in its character in all normal individuals ; 

 hence any deviation may be fairly regarded as indicating 

 a pathological condition of colour vision. As physiologists 

 we cannot admit as a valid argument against the test 

 that it is difficult or impossible satisfactorily to correlate 

 the physical facts with the physiological manifestations. 



NO. 2I0I, VOL. 82] 



But is it necessary or advisable that the divergence of 

 opinion on the exact conditions of Mr. Trattles's colour 

 vision should be further emphasised? The case has been 

 adjudicated upon, and has amply demonstrated the need 

 of reform either in the tests or in the application of the 

 tests. The Board of Trade cannot, and ought not, to 

 accept any risk of allowing a man with possibly dangerous 

 defective colour vision to pass the examination. It is 

 better that slight hardship should fall upon a few in- 

 dividuals than that many lives should be endangered ; but 

 the hardship must be minimised, and this object will be 

 best attained by ensuring that the individuals shall be 

 eliminated as early as possible in their careers. Hence it 

 is of the utmost importance that the first examination shall 

 be authoritative and conclusive. 



The Writer of the .\rticle. 



Records of the Earthquake of January 22. 



On January 22 the Kew Milne horizontal pendulum 

 recorded a large earthquake, of which the preliminary 

 tremors commenced at about Sh. 52.6m. a.m., and the 

 large waves at about 8h. 56.1m. The limits of registra- 

 tion, 17 mm., were exceeded between 8h. 58m. and 

 8h. 59m., and again at gh. 0'4m, Synchronous with both 

 maxima there were burrs on the magnetic declination trace. 

 The largest, which simulated an oscillation of 1-5' in 

 declination, commenced about 8h. 57m., and lasted about 

 five minutes. The movements on the declination trace were 

 unquestionably seismic in character, and represented the 

 mechanical effect of the principal earth tremors. The 

 horizontal and vertical force traces were not sensibly 

 affected. Charles Chree. 



The National Physical Laboratory (Kew Observatory 

 Department), Richmond, Surrey, January 27. 



An Earthquake Phenomenon. 



K CURIOUS phenomenon connected with the earthquake 

 of January 22 was that the maximum movement was 

 accompanied by a sudden tilt. The amount of this was 

 approximately one second of arc, its direction being towards 

 the north-west. This would correspond to a rise of the 

 ground on the south-east. It took place at about 8 a.m., 

 when the booms of five horizontal pendulums were suddenly 

 displaced from their normal position. Those oriented east 

 and west were swung to the north, whilst those at right 

 angles moved to the west. Pendulums in rooms 80 yards 

 apart were displaced similarly. From 12.45 they 

 crept back somewhat intermittently towards their original 

 position, which they reached about 4 p.m. 



Whether this indicates a local change in the dip of the 

 strata (chalk) on which my instruments are founded or 

 a more extended change of level cannot be stated with 

 any certainty. At Bidston a pendulum oriented east-west 

 was displaced towards the north, and at West Bromwich 

 a pair of pendulums swung more on one side of their 

 normal position than on the other. Their behaviour sug- 

 gested a displacement similar to that observed at Shide. 

 Permanent changes in the ground near the origin of a 

 large earthquake are common. J. Milne. 



Shide, Isle of Wight. 



The Preparation of Silicon. A Warning, 

 A COMMON method of preparing silicon is by mixing 

 silica and magnesium powder in molecular proportions and 

 heating until the following reaction takes place : — ■ 



SiO,4-2Mg = Si + 2MgO. 



The majority of text-books recommend the use of silica 

 in the form of silver sand or ground quartz, but they do 

 not say it is absolutely necessary, or even desirable, that 

 it should be in this form, and one well-known treatise 

 states that precipitated silica and pure magnesium powder 

 will yield very pure silicon. This authority adds that if 

 the experiment is performed with precinitated silica the 

 reaction is attended by a brilliant flash of light. 



