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KNOVV^LEDGE ♦ 



[March 9, 1883. 



Never read by firelight in the position shown in Fig. 5. 

 Myopia, or short sight, is often produced, particularly in 

 young people, by reading in an imperfect light. 



On Light. 



Now I must say a few words about light. 



Light is the cause of all colour. Colour is only a sensa- 

 tion in the brain, caused by a particular kind of light 

 being reflected from an object into the eyes. We say 

 trees are green, but they would not be green unless they 

 were lighted by a light which contained green rays. This 

 can be proved by a simple, yet perfectly convincing, experi- 

 ment. 



Place several pieces of paper of various bright colours 

 on a large piece of white paper, taking care to avoid the 

 use of yellow. Now illuminate these with a spiriWamp 

 which has had salt sprinkled on the wick ; the whole of 

 the coloured papers will appear grey. 



Colocr-Blindness. 



About one person in every twenty-five is to some extent 

 colour-blind, that is, cannot distinguish accurately between 

 colours. In extreme cases such persons cannot distinguish 

 between red and green. This defect often exists without 

 being suspected. The worst case I have ever known was 

 that of a workman who had been for years in my employ. 

 One day, I gave him a number of photograpiied stereo- 

 scopic slides to sort into two lots — one coloured, and the 

 other plain. Soon after he had sorted them I examined 

 them. They were divided into two lots, composed almost 

 equally of coloured and plain slides mixed together. This 

 induced me to test his eyes for colour-blindness, and I 

 found, to my surprise, that he could not tell the difference 

 between a piece of black cloth and a piect- of scarlet. 



This defect may be of the greatest importance. 



Both on railways and on board of ships, lamps with 

 coloured glasses are used for signalling at night and Hags 

 in the daytime. xVny sailor, guard, or engine-driver 

 sulFering from colour-blindness might be the cause of a 

 fatal accident by mistaking the colour of the signal shown. 



All such persons should liave their sight tested. 



I have contrived a spectroscope in which there is a com- 

 plete riband or rainbow of colour. Now, I can shut out 

 all but a small portion of this coloured rainbow, and allow 

 only a small strip of any particular colour to appear. The 



person whose sight is being tested is then asked to name 

 the colour that is visible. 



An easy way of testing the sense of colour is to give a 

 person two or three skeins of Berlin wool of different 

 colours. Then give them a bundle of wool of mixed colours, 

 and ask them to match the colours of them. 



This test is not to be compared to that with the spectro- 

 scope for accuracy. 



But under certain circumstances, even persons whose 

 colour-sense is most acute and accurate may be deceived as 

 to colour. 



{To he continued.) 



BUTTERFLIES I:N WINTER. 



REFERRING to the recent notice in Knowledge of a 

 butterfly in winter, it may interest many readers to 

 learn that such an occurrence is by no means rare, and 

 results, in most cases, not from unusually warm weather 

 forcing the insects to leave the chrys-alis, Vmt from the fact 

 that several of our butterflies hibernate in the winged 

 state, and may frequently be found torpidly hanging 

 from roofs, in hollow trees, and other sheltered spots, the 

 warm sunshine occasionally tempting them from their 

 hiding-places. 



Of our native species, ten, including the Peacock 

 ( Vanessa lo), the Small and Great Tortoiseshells ( V. 

 UrticcK anA PoJychloros), the Red Admiral (V. Atalanta), 

 and the Brimstone {G. Rhamni), thus pass the winter. Of 

 the others, some hibernate in the egg state, some as cater- 

 pillars, and the rest in the chrysalis. Those passing the 

 longest period in the latter stage are the noble Swallow- 

 tail (P. Jfachao?i), and the lovely little Orange-tip (A. 

 Cardamines), which become chrysalids in July or August, 

 and remain so until the following May, no amount of frost 

 or snow having any injurious effect upon them, but rather 

 tending to protect them from the attacks of slugs and other 

 enemies that in a damp mild winter do terrible damage ; 

 and experience proves that severe weather is sure to be 

 followed by a more than usual abundance of insects. 



Our general want of know^ledge on the subject is evident 

 in the last words of the familiar nursery doggerel : — ^ 



I 'd be a butterfly, born in a bower, 

 Christened in a teapot, and died in an hour. 



For a study of the life-history of these insects shows that 

 our shortest-lived species has a much longer natural exist- 

 ence, in the winged state, than the foregoing verse implies. 

 I recollect, one Christmas day, seeing five peacock butter- 

 flies chasing each other, and floating gracefully in the sun- 

 shine ; they were at least three months old, having emerged 

 from the chrysalis in August or September, and would — 

 no accident befalling them — live on until May or June, 

 when the females, with a never-erring instinct, would lay 

 their eggs on the leaves of the common stinging-nettles, the 

 natural food-plant of the future caterpillars, and having 

 thus fulfilled the last duty of their harmless lives, would 

 quickly go the way of all living things, leaving the next 

 generation to gladden our eyes with a reproduction of the 

 marvellous beauty of their parents. 



Martin J. Harding. 



Electric Lighting in Russia. — The Edi.<;on system is 

 in use in the saw-mill of M. Johnson at Yvaskylii, in 

 Russia. Forty lamps and a dynamo constitute the plant 

 at present. This is, thinks the Electrician, the most 

 northerly point to which electric lighting has penetrated 

 as yet ; it is between the 62nd and 63rd degrees of latitude. 



