588 



KNO^A/'LEDGE 



[April 21, 1882. 



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"Id knowled^a, (bat man only is to bo contemned and despked who is not in a 



tUt* of transition Nor is there anything more adverse to accuracy 



than fixity of opinion." — Faraday. 



" There is no harm in makinp a mistake, but great harm in making none. Show 

 me a man who makes no mistakes, and I will show yoa a man who has done 

 nothing." — Liebiq, 



" Ood's Orthodoxy is Truth."— C^aWcjt Xingslcy. 



(^\K CoiifSponlJfnct Columnsf. 



CADDIS-WORM CASES— INTELLIGENCE [IN ANIMALS. 



[385]. — I notice caddis-worms use, among other things, little 

 pieces of plants. I further notice, so long as the caddis has need of 

 them, so long do they keep tlieir life and original colour. How does 

 the worm keep them alive ? 



What do scientific men understand by reason ? A good definition 

 wanted. Why divided into ])0sitive and abstract ? What acts in 

 man arc the result of reasoning powers ? Is reasoning power 

 necessary for man to perform the acts of daily life ? Would the 

 acts performed by animals, if performed by men, be considered to 

 require roaaoning powers ? John Alex. Ollard. 



DOES THE MIXTURE OF BLUE AND YELLOW MAKE 

 GREEN LIGHT? 



[88G] — If your correspondent on this subject (April 7, p. 49G) 

 would consider the experiment referred to by Helmholtz for mixing 

 the lights rellooted from two-coloured spots by means of a piece of 

 polished glass held upright between them, he would quickly per- 

 ceive that the imago is produced by half the light from ono of the 

 spots, mingled with half the light from the other. But this is not 

 the only way of mixing differently-coloured lights on the retina. 

 It is easy, by means of a lens, to throw together the yellow and 

 blue (that is the ultramarine blue) prismatic rays, and to see that 

 they do not make green, but neutralise each other perfectly. The 

 persistence of the sensations excited by light on the retina enables 

 us also to produce the same effect by rapid rotation of a circular 

 disc, painted half with lemon yellow and half with French blue. 



In his fact 1, the yellow light from the sodium salt makes, with 

 the floa-grocn or verdigris light from the chloride of copper, a 

 yellowish-green mixture, which agrees with the theory ho 

 controverts. 



In his fact 2 and fact 3, the greenish colour which he notices 

 arises no doubt from the circumstance that the flames from the 

 blowpipe and from the Bunsen burner give out, not a pure or ultra- 

 marine bine, but a sea-green blue light. When the yellow from the 

 heated wire, or from the incandescent particles of carbon, is seen 

 through these flames, the result is a greenish appearance, which is 

 again consistent with the theory. 



In his fact 4, as described, it seems utterly inexplicable how the 

 green streak was prodnred in the prism j but as the sunbeam was 

 admitted between two laths of a Venetian blind, and Venetian 



blinds are commonly painted green, I suspect the lens must have 

 thrown an imago of a bright edge of a green lath across the pri/im. 



W. Bkn- . 



COLLISIONS AT SEA. 



l3H7J -With reference to Mr. Stewart Harrison's signals, :, I 



to in the article on Collisions at Sea last week, may I mcntii" i' 

 by Article 19 of Rule of the Road, steamers are already at ur . itv 

 to signal their intentions to each other by means of their sio.im 

 whistles, as follows : — 



Ono short blast — " I am directing my course to starboard," i.«. 

 to my right. 



Two short blasts — " I am directing my coarse to port," i.e. .t 

 left. 



Three short blasts — " I am going fall speed astern." 

 It would not be safe to use any signals asking another ship to alter 

 her course in any way, as the signal might be obeyed by the %Trong 

 ship — if two or three were in company, in an cstuarj- or channel — 

 and they might get mixed. Sailors woald be glad to be able to 

 avoid collision, though yon remark on their apparent unconcern 

 in the matter. They know how diflScnlt it would be to enforce 

 obedience to rules requiring certain signals to be made, and how 

 confusing it would be if those signals were not made when 

 expected. A frequent cause of collision is that the ship A, 

 which ought to give way, keeps on too long, and frightens 

 the ship B, which ought to keep her course, into acting upon 

 Article 23, and swerving, in order to avoid what she considers 

 immediate danger of collision ; perhaps starboarding at the very 

 instant that A " ports," and that, too, at the verj- last moment ; so 

 that there is no time to remedy the mistake. The blame ought, of 

 course, to lie with A, who, though he certainly did give way, did 

 not do so in time to let B know he was doing it. I have seen this 

 state of things from the deck of a small sailing-vessel, when I have 

 not dared to alter my course for fear of baulking the steamer, and 

 when the steamer has carried on, and " shaved" me, her tall sides 

 actually taking the wind out of our sails as she cut her wav close 

 by ; and yet, had we altered our course, we might have run right 

 under her bows. I have also seen the same thing from the opposite 

 point of view, viz., from a steamer's bridge, the officer in charge 

 apparently thinking that, so long as he knows that he intends to 

 keep clear, it is of no consequence whether those in charge of the 

 other ship get scared or not. There are practical difficulties in the 

 way of ])lacing the red and green lights as suggested in your 

 article ; in a sailing-ship, the sails and rigging would to a greater er 

 less extent mask the midship light. A streak of luminous paint all 

 round the hull, in addition to the present side lights, has been pro- 

 posed, and probably would better show small changes of course at 

 night than anything else. F. C. G. 



REPLIES TO QUERIES. 



Adjusting EqrATORiii.. — In taking the declination of a star 



six hours from the meridian, for the purpose of adjusting an 



equatorial, the refraction for its altitude at that instant should be 



taken out of Bessel's, or some other table. We must, though, of 



course, know what that altitude is, and this we find simply by 



adding the logarithm sine of the latitude to the logarithm sine of 



the star's declination. Thus, what, let us say, is the altitude of a 



Cvgni when six hours from the meridian in latitude 51° 30'? 



51° 30' lat. Sine 9 893645 



41° 52' dec. of a Cygni. Sine 9S48172 



33° 31' 11" Sine 9742117 



Turning now to any table of refractions, we find that the mean 

 refraction corresponding to 33° 31' is 1' 33'9''. Of course, part of 

 this operates in shifting the star in right ascension; but we may 

 use it all for our present purpose without introducing any error 

 likely to be sensible in our instrumental adjustments. 



[350] — "Whitby "is informed that jet is nothing but a com- 

 pact variety of coal, and has been formed in precisely the same 

 way. Its vegetable structure is readily seen, in thin sections, 

 under the microscope. F.R.A.S. 



The number of asteroids that have been discovered is now 220. 

 Recent researches by Herr Hornstein (communicated to the Vienna 

 Academy) appear to prove that the number of those with a diameter 

 of over twenty-five geographical miles is extremely small, and that 

 probably all such were discovered before 1850. On the other hand, 

 the number of asteroids with a diameter less than five miles seems 

 also to be very small, at least in the parts of the asteroid zone next 

 Mars; in the outer regions next Jupiter there may be a more con- 

 siderable number of these very small bodies. Most asteroids seem 

 to have a diameter of between five and fifteen miles. 



