538 



NATURE 



[April 6, 189^ 



THE SENSITIVENESS OF THE EYE TO 

 LIGHT AND COLOUR} 



np] 



HERE may be some here who have had the pleasure 

 -or the pain — of rising very much betimes in a 

 Swiss centre of mountaineering in order to gain some 

 mountain peak before the sun has had power enough to 

 render the intervening snow-fields soft, or perhaps dan- 

 gerous. Those who have will recollect what were the 

 sensations they experienced as they sallied out of the 

 ■comfortable hotel, after endeavouring to swallow down 

 breakfast at 2 a.m., into the darkness outside. Perhaps 

 the night may have been moonless, or the sky slightly 

 overcast, and the sole light which greeted them have been 

 the nervous glimmer of the guides' lanterns. By this 

 feeble light they may have picked their way over the 

 stony path, and between the frequent stumbles over some 

 half hidden piece of rock lying in the short grass they 

 may have had time to look around and above them, and 

 notice that the darkness of the night was alone broken 

 by stars which gave a twinkle through a gap in the 

 clouds, or if the sky were cloudless, every star would be 

 seen to lie on a very slightly illuminated sky of trans- | 

 parent blackness. Although giant mountains may have 

 'been immediately in front of them, their outlines would 1 

 be almost if not quite invisible. As time went on the sky I 

 would become a little brighter, and what is termed 'Ca.&petit [ 

 jour would be known to be approaching. The outlines 

 of the mountains beyond would become fairly visible, the 

 tufts of grass and the flowers along the path would still ; 

 be indistinguishable, and most things would be of a cold 

 grey, absolutely without colour. The guide's red woollen j 

 scarf which he bound round his neck and mouth would 

 ■be black as coal. But a little more light, and then some 

 flowers amongst the grass would appear as a brighter 

 grey, though the grass itself would still appear dark ; but 

 that red scarf would still be as black as a funeral gar- 

 ment. The mountains would have no colour. The sky 

 would look leaden, and were it not for the stars above it 

 might be a matter of guesswork whether it were not 

 covered over with cloud. 



More light still, and the sky would begin to blush in 

 the part where the sun was going to rise, and the rest 

 would appear as a blue-grey ; the blue flowers will now 

 be blue, and the white ones white ; the violet or lavender 

 coloured ones will still appear of no particular colour, and 

 the grass will look a green grey, whilst the guide's neck- 

 gear will appear a dull brown. 



The sun will be near rising, the white peaks beyond 

 will appear tipped with rose ; every colour will now be 

 distinguished, though they would still be dull ; and, 

 finally, the daylight will come of its usual character, and 

 the cold grey will give place to warmth of hue. 



But there may be others who have never experienced 

 this early rising, and prefer the comfort of an ordinary 

 English tramp to that just described ; but even then they 

 may have felt something of the kind. In the soft autumn 

 evening, when the sun has set, they may have wandered 

 into the garden and noticed that flowers which in the day- 

 time appear of gorgeous colourings— perhaps a mixture 

 of red and blue — in the gloaming will be very different in 

 aspect. The red flowers will appear dull and black ; a 

 red geranium, for instance, in very dull light, being a 

 sable black, whilst the blue flowers will appear whitish- 

 grey, and the brightest pale yellow flowers of the same 

 tint ; the grass will be grey, and the green of the trees the 

 same nondescript colour. A similar kind of colouring 

 will also be visible in moonlight when daylight has 

 entirely disappeared, though the sky will have a trans- 

 parent dark blue look about it, approaching to green. 

 These sensations, or rather lack of sensations of light and 



1 A L:cture dilivareia the Roval Institution of Great Britaia by Cap ain 

 W. de W. Abney, C.B., R.E., D.C.L., F.R.S. 



NO. 122^. VOL. 47I 



colour, which as a rule attract very little attention, as 

 they are common ones, are the subjects of my discourse 

 to-night. 



Experiments which can be shown to a large audience 

 on this subject are naturally rather few in number, but I 

 will try and show you one or two. 



We are often told that the different stages of heat to 

 which a body can be raised are black, red, yellow, and 

 white heat, but I wish to show you that there is an inter- 

 mediate stage between black and red heat, viz. a grey 

 heat. An incandescent lamp surrounded by a tissue 

 paper shade, has a current flowing through it, and in this 

 absolutely dark room nothing is seen, for it is black hot. 

 An increase of the current, however, shows the shade of 

 a dim grey, whilst a further increase shows it as illum- 

 inated by a red, and then a yellow light. A bunch of 

 flowers placed in the beam of the electric light shows every 

 colour in perfection ; the light is gradually dimmed down, 

 and the reds disappear, whilst the blue colours remain and 

 the green leaves become dark. These two experiments 

 show that there is a colour, if grey may be called a colour, 

 with which we have to reckon. 



Now the question arises whether we can by any means 

 ascertain at what stage a colour becomes of this grey hue, 

 and at what stage of illumination the impression of mere 

 light also disappears, and whether in any case the two 

 disappear simultaneously. 



As all colours in nature are mixed colours, it is at the 

 outset useless to experiment with them in order to arrive 

 at any definite conclusion, hence we are forced — and the 

 forcing in this direction to the experimentalist is a very 

 agreeable process — we are forced to come to the spectrum 

 for information. 



The apparatus on this table is one which I have before 

 described in this theatre, and it is needless for me to 

 describe it again. I can only say that it has in all 

 colour investigations been of such service that any 

 attempt on my part to do without it would have been 

 most disadvantageous. The apparatus enables a patch 

 of what is practically pure monochromatic light of any 

 spectrum colour to be placed upon the screen at once, 

 and an equally large patch of white light alongside it, 

 by means of the beam reflected from the first surface of 

 the first prism. 



It should be pointed out that this beam of white light 

 reflected from the first prism of the apparatus, having 

 first passed through the collimator, must of necessity 

 diminish with the intensity of the spectrum, when the 

 collimator slit is closed. 



Having got these patches, the next step is to so en- 

 feeble the light that their colour and then their visible 

 illumination disappear. 



An experiment which well demonstrates loss of colour 

 is made by throwing a feeble white light on one part of 

 the screen, and then in succession patches of red, green, 

 and violet alongside it. The luminosity of the coloured 

 light gradually diminishes till all the colour disappears, 

 the white patch being a comparison for the loss of 

 colour. 



If red, green, and violet patches be placed alongside 

 each other, and-they are bedimmed in brightness together, 

 it will be noticed that the red disappears first, then the 

 green, and then the violet ; or I may take a red and 

 o-reen patch overlapping, which when mixed form orange, 

 and extinguish the colour : the slit allowing red light to 

 fall on the screen may be absolutely closed, and no altera- 

 tion in the appearance of the patch is found to occur. 

 This shows, I think, that when all colour is gone from a 

 once brilliant colour, a sort of steel-grey remains behind, 

 and that red fails to show any luminosity when the green 

 still retains its colour. 



The measurement of the extinction of colour from the 

 different parts of the spectrum was made on these prin- 



