Septembek I, 1 910] 



NATURE 



26: 



3binch Lick, using a power of 



2Sinch Greenwich ,, ,, 



i8A-inch Deaibofn ,, „ 



S-inch (Maw) ,, ,, 



6-inch (Solii) ,, ,, 



1800 

 1400 

 950 

 400 



3C0 



Atmospheric conditions affect tlie large glasses much 

 lucre than the smaller, for we find in actual practice that 

 the Lick observers prefer powers of 1000 and 1500 ; the 

 Greenwich observers prefer (570 and 1120; Hough, with the 

 iJSJ-inch, used generally a power 390, and less frequently 



Maw uses powers of about 300 and 400 on both his 

 6-inch and 8-inch, w-hile Soli uses 350 on his 6-inch. 



When an observer is quite used to his instrument and his 

 eye-pieces, he develops a preference for one particular eye- 

 piece under most all conditions. 



One element, as yet not mentioned, has naturally a 

 great influence in the clioice of an object-glass, viz. the 

 range of visibility, or the ability to show faint objects. 

 The above remarks apply to pairs the components of which 

 are fairly equal ; but, in general, distant companions are 

 very faint. 



The light-grasping power of a telescope depends on the 

 surface or diameter squared of the object-glass. A good 

 i-inch object-glass should show a ninth-magnitude star, 

 and one star is said to be a magnitude fainter than another 

 ■n-lien its light is 2-5 times less. 



Consequently, the aperture must be v'2'5 greater to show 

 it. Roughly, v'2'5=i-6, and hence if i inch shows a ninth 

 magnitude i x i-6, or a i-6-inch shows a tenth magnitude, 

 or generally 



Star masnitude q 10 11 12 13 



Aperture in inches 10 16 2'5 40 6'3 



Of course, this table is not to be taken too serio 



jsly, as 

 already 



it is governed by much the same conditions a 

 mentioned for separating power. Bear in mind Burnham's 

 words : " An object-glass of inches one night will show 

 the companion to Sirius perfectly ; on the next night, just 

 as good in every respect, so far as one can tell with the 

 unaided eye, the largest telescope in the world will show 

 no more trace of the small star than if it had been blotted 

 out of existence." 



T hope, with a little twisting and adaptation, the fore- 

 going remarks may be made to answer the fundamental 

 principles underlying the apparently easv questions. 



Mr. Scholes is quite right as to the glare, and the larger 

 aperture by increasing the separation, and by making the 

 apparent discs smaller, does make observation easier. 



1. Lewis. 



Colour-v.sicn. 



\s one who was responsible for the testing for colour- 

 ^ -inn of several thousands of drivers and firemen, 1 

 should like to refer to the method of testing by means of 

 different coloured skeins of wool. 



The usual method is to take a particular skein of wool 

 and request the person who is being tested to select in 

 succession the three or more skeins which mostly resemble 

 it. In some cases I found that men who were clearly 

 colour-blind succeeded in passing such a test satisfactorily. 



It must be remembered that a colour-blind person has 

 been accustomed to consider his capacity for appreciating 

 colour differences in the light of other people's statements. 

 It thus comes about that they learn to consider differences, 

 which are really colour differences to those whose sight is 

 normal, as being partly due to intensity of light, texture, 

 or other considerations. They are aware, of course, that 

 they cannot always detect differences of colour in the 

 ready way that others can, but they also feel that they 

 can often see differences much more quickly than can 

 others. With the colour-blind, therefore, the capacity for 

 matching or naming colours becomes more and more 

 perfect the greater their experience becomes of the objects 

 to be compared. Now, in the case of the w'ool test, the 

 different coloured skeins are certainly in many cases of 

 different texture, coarsen1;ss, or gloss. The skeins are 

 also frequently numbered. With a little careful study of 

 the wools with which the tests are carried out, it may be 

 NO. 2 13 I, VOL. 84] 



quite possible for a colour-blind man to get through the 

 tests satisfactorily unless great care is exercised. 



I found the following to be a ready method of detecting 

 colour-blindness. The wool skeins were arranged in the 

 order of their brightness, the white skein being at one end 

 and the black at the other. It is, of course, somewhat 

 difficult to estimate the comparative brightness of a red 

 and a yellow object. I found, however, that with a little 

 practice and care this could be done satisfactorily. If a 

 person whose vision is normal be asked to pick out the 

 darkest skeins, he will at once pick the black one and 

 afterwards those next to it. On the other hand, a colour- 

 blind person will probably pick the black skein first and 

 then the reds or greens, the darker shades being selected 

 first. A test of this kind is most striking. In one case, a 

 man who had got through the ordinary tests with some 

 hesitation selected all the reds before the dark greys, 

 neutral tints, greens, &c., although some of the reds were 

 much brighter colours than the greens. 



The better plan is to take a number of different coloured 

 skeins of wool and ask the person who is being tested to 

 arrange them in their order of brightness. A markedly 

 colour-blind person cannot do this properly. 



R. M. Defxey. 



Melbourne House, Osmaston Roid. Derbv. 



LAKE EDWARD, RUWENZORI. AND THE 

 VGAX DA-CON GO FRONTIER. 

 'X'HE argument lately arrived at by the representa- 

 •*■ lives of Great Britain and the Congo has 

 affected the settlement of a troublesome boundary dis- 

 pute, in which the competence of anv diplomacy to 

 deal with a geographical question in a scientific 

 manner has not shown itself in a particularly favour- 

 able light. 



The original agreement, the foiis et origo of all the 

 subsequent mischief, was signed at Brussels on May 9, 

 1894. By this it was enacted : — 



" That the sphere of influence of the Independent Congo 

 State shall be limited to the north of the German sphere 

 in East .\frica by a frontier following the thirtieth meridian 

 east of Greenwich up to its intersection by the watershed 

 between the Nile and the Congo, and thence following 

 this watershed in a northerly and north-westerly direc- 

 tion." 



At the time this agreement was made the 30th 

 meridian was shown on the maps as dividing Lake 

 Edward into two approximately equal parts, and as 

 passing to the west of the whole Ruwenzori range. It 

 is, however, a commonplace among geographers and 

 surveyors that a determination of longitude in an un- 

 surveyed country is liable to large errors, and that a 

 meridian line is, of all possible boundaries, the worst 

 that can be selected. In this case the actual event 

 proved that the selection of this line had resulted in 

 the maximum of inconvenience and loss. The true 

 position of the meridian was found to be about half 

 a degree east of its position as assumed in 1894, and 

 a strict interpretation of the letter of the treaty would 

 have involved our retirement from Lake Edward and 

 from practicallv the whole of the Ruwenzori district. 

 Such a contiogency was obviously intolerable, and the 

 only practicable course was to arrive at some sort of 

 compromise which should, as far as possible, mini- 

 mise our loss. The commissioners entrusted with the 

 recent negotiations arrived at what perhaps was the 

 best solution available at this date, and by surrender- 

 ing to the Congo the whole of the north shore of 

 Lake .Albert, they resrained the eastern half of Lake 

 Edward, and about half Ruwenzori. The net result 

 of the whole transaction is therefore that we lose all 

 the country lying between Lake .Albert and the Congo- 

 Nile watershed and the western half of the Ruwenzori 

 range. 



From the geographical point of view the great error 



