498 



NA rURE 



[March 24, 1887 



whom he met in the Monbutlu country, and now Dr. Liulwig 

 Wolf, who, with Wissmann, recently explored the Sankuru, the 

 great southern tributary of the Congo, gives us many details of 

 a similar pygmy race among whom he sojourned for some time, 

 in the district to the north-we^t of the station Luluaburg. He 

 found entire villages inhabited by tiny men and women, of a 

 height of not more than i '40 metre. Among their neighbours 

 they are known as Batua. These are nomad tribes devoting 

 themselves exclusively to the chase and the manufacture of palm 

 wine. Their villages, consisting of hut", are met with in 

 clearings in the forests which cover the greater part of the 

 country. Each district thus possesses a village of pygmies. As 

 is the case of the Akkas among the Monbuttus, so the Batua 

 among the Bakubu are regarded as little benevolent beings 

 whose special mission is to provide the tribes among whom they 

 sojourn with game and palm-wine. In exchange, manioc, 

 maize, and bananas are given to the pygmies. Generally 

 they live apart, but sometimes they unite themselves with 

 races of larger stature. They excel in the art of scaling palm- 

 trees to collect the sap, and in setting traps for game. Their 

 agility is almost incredible. In hunting they bound through 

 the high grass like grasshoppers, facing the elephant, antelope, 

 and buffalo with the greatest audacity, shooting their arrows 

 with rare precision, following up rapidly with a stroke of the 

 lance. Physically the Batua are very well made, having abso- 

 lutely no deformity. They are simply little men, well pro- 

 portioned, very brave, and very cunning. Their mean height 

 is I '30 metre. Their skin is a yellow-brown, less dark than 

 that of larger races. Their hair is short and woolly. Neither 

 the Akkas nor the Batua have any beard. 



SUNLIGHT COLOURS'- 

 CUNLIGHT is so intimately woven up with our physica' 

 enjoyment of life that it is perhaps not the most unin- 

 teresting subject that can be chosen for what is — perhaps 

 somewhat pedantically — termed a Friday evening "discourse." 

 Now, no discourse ought to be possible without a text on which 

 to hang one's words, and I think I found a suitable one when 

 walking with an artist friend from South Kensington Museum 

 the other day. The sun appeared like a red disk through one of 

 those fogs which the east wind had brought, and I happened to 

 point it out to him. He looked, and said, ' ' Why is it that the sun 

 appears so red ? " Being near the railway station, whither he 

 was bound, I had no time to enter into the subject, but said if 

 he would come to the Royal Institution this evening I would 

 endeavour to explain the matter. I am going to redeem that 

 promise, and to devote at all events a portion of the time 

 allotted to me in answering the question why the sun appears 

 red in a fog. I must first of all appeal to what everyone who 

 frequents this theatre is so accustomed, viz. the spectrum ; I am 

 going not to put it in the large and splendid stripe of the most 

 gorgeous colours before you with which you are so well ac- 

 quainted, but my spectrum will take a more modest form of 

 purer colours some twelve inches in length. 



I would ask you to notice which colour is most luminous. 

 I think that no one will dispute that in the yellow we have the 

 most intense luminosity, ancl that it fades gradually in the red on 

 the one side and in the violet on the other. This then may be 

 called a qualitative estimate of relative brightnesses ; but I wish 

 now to introduce to you what was novel last year, a quantitative 

 method of measuring the brightness of any part. 



Before doing this I must show you the diagram of the ap- 

 paratus which I shall employ in some of my experiments. 



RR are rays (Fig. i) coming from the arc light, or, if we were 

 using sunlight, from a heliostat, and a solar image is formed by 

 a lens, l-j, on the slit Sj of the collimator c. The parallel rays pro- 

 duced by the lensLj ai* partially refracted and partially reflected. 

 The former pass through the prisms PiP„, and are focused to 

 form a spectrum by a lens, 1,3, on D, a mov.able ground glass 

 screen. The rays are collected by a lens, Lj, tilted at an angle 

 as shown, to form a white image of the near surface of the second 

 prism on F. 



Passing a card with a narrow slit, Sj, cut in it in front of the 

 spectrum, any colour which I may require can be isolated. The 

 consequence is that, instead of the white patch upon the screen, 

 I have a coloured patch, the colour of which I can alter to 



' Lecture delivered by Capt. W. de W. Abney, R.E., F.R.S., at the 

 RoysJ Institution, on Febraary 25, 1887. 



.any hue lying between the red and the violet. Thtis, then, 

 we are able to get a real patch of very approximately homo- 

 geneous light to work with, and it is with these patches 

 of colour that I shall have to deal. Is there any way of 

 measuring the brightness of these patches ? was a ques- 

 tion asked by General Festing and myself. After trying 

 various plans, we hit upon the method I shall now sliow 

 you, and if anyone works with it he must become fascinated with 

 it on account of its almost childish simplicity— a simplicity, I 

 may remark, which- it took us some months to find out. Placing 

 a rod before the screen, it casts a black shadow surrounde<l with a 

 coloured background. Now I may cast another shadow from a 

 candle or an incandescence lamp, and the two shadows are 

 illuminated, one by the light of the coloured patch and the other 

 by the light from an incandescence lamp which I am using to- 

 night. [Shown.] Now one stripe is evidently too dark. By 

 an arrangement which I have of altering the resistance inter- 

 posed between the battery and the lamp, I can diminish or 

 increase the light from the lamp, first making the shadow it 

 illuminates too light and then too dark compared with the other 

 shadow which is illuminated by the coloured light. Evidently 

 there is some position in which the shadows are equally 



%f' 





^1 



Fi(i. 1. — Colour Photometer. 



luminous. When that point is reached, I can read off the 

 current which is passing through the lamp, and having pre- 

 viously standardised it for each increment of current, I know 

 what amount of light is given out. This value of the in- 

 candescence lamp I can use as an ordinate to a curve, the 

 scale number which marks the position of the colour in the 

 spectrum being the abscissa. This can be done for each part 

 of the spectrum, and so a complete curve can be constructed 

 which we call the illumination curve of the spectrum of the light 

 under consideration. 



Now, when we are working in the laboratory with a 

 steady light, we may be at ease with this method, but 

 when we come to working with light such as the sun, 

 in which there may be constant variation owing to p.assing, 

 and maybe usually imperceptible, mist, we are met with a 

 difficulty ; and in order to avoid this. General Festing and myself 

 substituted another method, which I will now show you. We 

 made the comparison light part of the light we were measuring. 

 Light which enters the collimating lens partly passes through 

 the prisms and is partly reflected from the first surface of the 

 prism ; that we utilise, thus giving a second shadow. The reflected 



