i66 



NATURE 



[July 5, 1917 



when the eye is unadapted. The light from a gas 

 lamp, blinding- to an eye accustomed to darkness, 

 appears fedale when viewed in full sunlight ; yet its 

 intensity of ultra-violet r£\diation is unchanged. Snow 

 blindness may be caused on dull days, presumably 

 owing to the inability of the upper part of the retina 

 to adapt itself to the unaccustomed light from the 

 ground. Dr. E. K. Martin ^ could detect no absorp- 

 tion by the refracting: media of the eye in the visible 

 spectrum, but he found that in the ultra-violet absorp- 

 tion begins at about 038 /x, and becomes complete for 

 0-35 IX and shorter wave-lengths. As the solar spec- 

 trum stops short at 029 /x (owing to the absorption 

 of the atmosphere), we see that there is a short region 

 from 035 /* to 029 Ik, which may be absorbed and pro- 

 duce some physiological action. Therefore in cases 

 where considerable sunlight or skylight must enter 

 the eye, it is a good precaution to use a filter which 

 will stop these ultra-violet rays, but at the same time 

 it must be remembered that the main symptoms 

 known as glare are not due to ultra-violet light at all, 

 but simply to an illumination too intense for an un- 

 adapted retina. 



To the normal eye the maximum luminosity in the 

 solar spectrum lies in the region near the yellow. A 

 great many filters now being supplied for anti-glare 

 purposes are of a yellowish colour. They transmit 

 80 per cent, or so of the red, yellow, and green, and 

 absorb the violet and ultra-violet fairly completely. 

 It would seem at first sight as if such, fiUers ' were 

 very desirable to eliminate possibly harmful rays, and 

 to reduce slightly the brightness of transmitted light. 

 On using the screen, however, the apparent brightness 

 of most objects seems to be increased, and glare is as 

 evident as before. The phenomenon of adaptation is 

 not yet fully understood, but we may refer in this 

 connection to the work of Broca and Sulzer,- who 

 found, in measuring the growth of visual sensation 

 with time, and using various colours, that in every 

 case there is an overshooting of sensation beyond its 

 final value. With blue light the maximum sensation 

 is at least five times, and with white light twice, the 

 final value. If, then, the removal of the shorter wave- 

 lengths interferes with adaptation, the apparent in- 

 crease of brightness when using a yellow filter is 

 explained. Obviously as an anti-glare glass the 

 screen is worse than useless. This does not apply to 

 the greenish glasses which actually reduce the appar- 

 ent luminosity. In passing, it may be noted that a 

 coloured filter of this nature is often useful for increas- 

 ing contrast, eliminating blue light from haze, and 

 increasing visual acuity. ' Amber ' and ' red ' filters 

 are sometimes used. These generally absorb the 

 green and blue parts of the spectrum, transmission 

 again beginning in the violet. If fairly deep in colour 

 they efTectually stop glare, and may be exceedingly 

 useful for special contrast work. 



Neutral-tinted glasses have a comparatively uniform 

 transmission over the whole spectrum. The use of 

 such filters in practice immediately reduces any glare, 

 and a proper balance of adaptation is established. It 

 is curious to note in this connection that many of these 

 neutral-tinted glasses, wb.ien about i mm. thick, 

 transmit the ultra-violet from 0-39 />i to 0-32 /x almost 

 as well as ordinary glass. They have often a slight 

 increase of transmission in the green, and in the 

 extreme red and infra-red they again show little ab- 

 sorption beyond that expected from glass of equivalent 

 thickness. Light transmitted by a bundle of such 

 glasses is found to consist of the extreme red, a little 

 green, and the extreme violet ; the sky, viewed through 

 the bundle, appears of a deep purple colour. 



1 Prf)C. Roy. Soc, B., Ixxxv., p. 319. 



2 Compi. rend., cxxxvii., 1903. 



NO. 2488, VOL. 99] 



Intense infra-red radiation also seems to produce 

 well-marked effects on the eyes when exposure i& 

 regular and long-continued, but here again the amount 

 ordinarily received by an eye exposed to daylight 

 appears to be incapable of causing harm. It is un- 

 certain how much (if any) of the discomfort of glare 

 from the ground experienced in bright sunshine is due 

 to the action of these rays. A correspondent of Sir 

 W. Crookes describes the effect produced by spectacles 

 of a glass containinp- ferrous iron, and absorbing the 

 infra-red, as producing a marked cooling effect on 

 the vision. This sensation may, of course, be merely 

 the result of the relief from glare which the blue- 

 green tint of the glass would secure. Manifestly,, 

 however, in circumstances where it may be necessary 

 to look directly at or near the sun, an efficient filter 

 is desirable to reduce intensity in all parts of the 

 spectrum. 



Some glasses show marked absorption in the infra- 

 red in the neighbourhood of i/x, notably those contain- 

 ing ferrous iron, cupric oxide, or ferric iron and cobalt 

 together ; these are blue to blue-green in colour. In- 

 teresting substitutes are metallic films on glass, suit- 

 ably protected, which, for the most trying conditions, 

 should make ideal glare glasses. As most of the 

 radiation would be reflected, the glasses would tend 

 to keep cool. 



The problems arising in industrial work are rela- 

 tively simple. For dealing with arc lamps, and 

 sources extremely rich in ultra-violet light, dark green 

 or brown-green glasses will be most suitable. For 

 welding operations efficient protection from ultra- 

 violet, visible light, and infra-red is important, as in 

 this case it is necessary to watch the source of light 

 intently. It is important also to select a filter which 

 \yill not greatly distort the colour of the emitted light, 

 as temperatures are judged in this way. A fairly dark 

 neutral glass of good thickness will generally answer 

 all practical purposes, but a highly efficient protector 

 could be made by depositing a silver film of suitable 

 thickness on a plate of heavy lead glass. This would 

 be, as previously suggested, very effective in reflecting 

 the heat rays, but the film would have to be protected. 



A gold film can be made to transmit green light 

 while reflecting almost all other radiations. Spec- 

 tacles of this nature could be used with great advan- 

 tage if it were necessary to work close to molten 

 metal, or in many other circumstances. 



It is hoped that this short summarv of the subject 

 may be of use to those dealing with problems in this 

 connection. An extended series of tests on certain 

 filters has been made by the present writer, and the 

 results are now in course of publication in the Proceed- 

 ings of the Optical Society. Filters for various pur- 

 poses can thus be chosen, but their m,ost important 

 test will be their efficiency in actual use. 



L. C. Martin. 



Electric Discharge from Scythe. 



In reference to Mr. Pannell's observations recorded ^ 

 in Nature of June 21 (p. 324), Mr. William Wilson 

 more than a hundred vears ago discovered that when 

 dry wood is chipoed with a knife the chips and knife 

 become oppositely electrified. 



There is no need of darkness to test the pheno- i 

 menon in the case of drv grass cutting with the scythe. 

 An ordinary gold-leaf electroscope held in the. mower's 

 hand, and having a wire attachment with the metal 

 of the scythe, would scarcely fail to give indications ; 

 if the electrification is actually sufficient to produce ! 

 disruptive discharges. Charles E. Benham. 



Colchester, June 23. 



