Ai'orsT, 1901.] 



KNOWLEDGE. 



177 



a concealed native would uot rise fi"oni behind a bush 

 in the line of tire at the monieut. 



We never actually shot anyone, however, although 

 there were many narrow shaves. The traps which we 

 set for mammals, although carefully concealed, seldom 

 brought us profit. Goats tramped over them, natives 

 stole them, presumably for the copper wire of which 

 they wore made, and dogs dragged them away for the 

 meat with which they were baited. 



These dogs, as is usual in Africa and the East, are 

 never fed. and consequcntlv become very bold and expert 

 thieves. I had tJie misfortune to be ill in our first campi, 

 and the dogs used to creep into my tent at night and 

 drink the milk at my be<lside. 



Another visitor, but a pleasing one. appeared in the 

 daytime. This was a little bird — the Lesser Whit«- 

 throat*^ — a summer visitor to England, and exceedingly 

 numerous during our winter and spring on the White 

 Nile. The Whitcthroat came to my tent for water, a 

 bucket of which was kept near my bed. On the very 

 first day of my illness this bird found out the water 

 and came into the tent, perching on the bed or my arm. 

 It stayed there most of the day, and whenever I 

 splashed my hand in the water it would hop down my 

 ami and suck the drops of water from my finger tips. 

 The bird was always thirsty, and although the river 

 was quite near it seemed to prefer to drink in this way, 

 and so I was amused by this delightful little bird all 

 through the heat of the day. I missed it much on 

 moving from this camp. 



The extreme dry heat of the counti-y affects the birds 

 perhaps more than other animals. All the land birds, 

 large and small, almost always kept their mouths open 

 as though gasping in the heat. Yet they sing, some of 

 them as beautifully as oiu' songsters, and seem as happy 

 as birds usually are. We used to take advantage of 

 their propensity to drink as a means to entice them 

 \s ithin range of the camera. A tin of water was placed 

 in the sand at a few yards distance from the camera, 

 which was kept focussed, and ready for action whenever 

 a bird came to drink. In this way many interesting 

 photographs were apparently secured, but alas, on 

 development, all my " photographs " proved complete 

 failures. Messrs. Dallmeyer had carefully fitted up for 

 me one of Messrs. W'atson iV Sons' " Gambler Bolton 

 cameras with telephoto lenses. The camera proved 

 strong enough to resist the heat and the camels, and 

 would no doubt have worked very well had I taken plates 

 and risked the great chance of their being broken. 

 I chose, however, to take films, which became seriously 

 affected by the great heat, and were, moreover, defective 

 in the making as regards the emulsion. For this 

 ruinous defect the maker desei-ves to be named. Such a 

 defect could, of course, be guarded against by testing 

 each batch of films befoi-e starting, but the fogging due 

 to the heat cannot be avoided. I think it wise to 

 mention this because, although I am well aware that 

 films arc often perfectly successful in very hot climates, 

 they are as often not, and the successes are well 

 advertised while the failures are seldom reported. Half 

 the number of plates in such a climate would bo more 

 certain to yield success if very carefully packed than 

 twice the number of films. 



Only the common and boldest birds came down to 

 the tin of water to drink and be photographed. Of 

 these a species of Bulbul,** a bird a little larger than 

 a Robin, with a black head, a brown back, and a white 



^ Sylvia curruca (Linn.) *• Pycnonotus arfinoe (irempr. it Eljr.) 



breast, was the most confiding and usually the first to ti-y 

 the experiment. One or two of these birds were always 

 in the trees over our camps, and their pleasing llute-likc 

 notes, almost exactly syllabled by the words " til-willow," 

 were continually to be heard. 



THE BRIGHTNESS OF STARLIGHT. 



I>y .J. E. Gore, k.k.a.s. 

 It is probably a matter of common observation that on 

 a clear moonless night it is never absolvitely dark, even 

 at midnight; a certain amount of light is given by the 

 stars. W^hat does this light amount to in terms say of 

 full moonlight; Miss Gierke, in her "System of the 

 Stars, " gives the light of all stars down to 9\ magnitude 

 as about l-80th of full moonlight. M. G. I'llcrmite 

 found starlight equal to 1-lOth of moonlight, but this 

 estimate is evidently too high. The difTercnce^ between 

 a bright moonlight night and one illuiuiiiatcd by star- 

 light alone is very considerable. 



Let us make an attempt to estimate the total amount 

 of stai-light by computing the light emitted by all the 

 visible stars down to the faintest point visible in the 

 largest telescopes, like those of the Yerkes and Lick 

 Observatories. The data available for this calculation 

 are rather uncertain, but an approximation to the truth 

 may perhaps be possible. 



To express the total amount of starlight in terms of 

 the light of a star of zero magnitude, like Arcturus, 

 and thence in terms of moonlight, let us assume — as is 

 now admitted by most authorities on the subject — that 

 the total number of the visible stars is about 100 

 millions. Let us also assume that the " light ratio " is 

 2.512 (now accepted by all astronomers) — that is, that a 

 star of zero magnitude gives 2.512 times the light of a 

 star of the 1st magnitude, a star of the 1st magnitude 

 2.512 times the liglit of a 2nd magnitude star, and so 

 on. To enable us to make this calculation it will be 

 necessary to estimate the number of stars of each mag- 

 nitude down to the 17th magnitude, which is about the 

 faintest visible in the great Yerkes telescope. Dr. Gould, 

 in his I'riiiiiiiiietrKi Arijiniiiia, gives the following 

 formula for computing the total number of stars visible 

 in both hemispheres to any given magnitude, m. 

 2,„ = 10051 X (3-9120)'" 



From this I find the following : 



MufJHU' inle. 



To ].0 iiiflusiv<^ 



„ ^•" 



„ 3.U „ 



., 4.0 



„ .5.0 



„ 6.0 



„ 7.0 ., 



„ 8 



„ 9-0 „ , 



From this it will be seen that the formula gives for. 

 the fainter stars numbers enormously too large. For 

 the brighter stars, viz., those of the 6th ^magnitude, the 

 numbers seem to be rather small. liouzeau, who ob- 

 served himself all the stars visible to the naked eye in 

 both hemispheres, gives the following figures: — 



MiiL'iii tilde. No. ofStiirs. 



1 20 



2 51 



3 200 



4 ... 595 



5 ... 1213 



6 . . ... 3640 



Total 



5719 



