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♦ KNOWLEDGE ♦ 



[JCNE 



" Let Knowledge grow from more to more." — Alfbed Tennyson. 



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The Editor is not responsible for the opinions of correspondents. 



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 AND DIRECTED ENVELOPE BE ENCLOSED. 



SINGLE VISION. 



[846] — Tonr correspondent, " Single Vision," in his very interest- 

 ing letter, correctly describes his own case as being one of extreme 

 disparity of focus between the two eyes. I do not recollect, in all 

 my experience of thirty years, such an extreme case. 



I find it is very seldom that, with such extremedisparity of focus, 

 the person can bear the focus being made equal by means of 

 lenses, but in some cases they can do so ; and I have visited a case 

 within the last few days in which the person wears a convex lens 

 of 16" focus on the left eye, and 40" on the right eye, with great 

 comfort and satisfaction. 



Strangely enough, the one point on which your correspondent 

 considers that he may speak with confidence, he may be mistaken 

 in. He is of opinion that " two well-defined images of different 

 dimensions will not coincide and coalesce." Now, if any person 

 having equal vision will take an ordinary spectroscope and place on 

 one side of it, in the centre of the field of view, a bronze-copper 

 halfpenny, and on the other side, in the centre of the field of view, 

 a bronze-copper pennypiece, although the two images given through 

 the lens will differ very greatly in size, a person using the spectro 

 will see an image clearly defined midway between the size of the 

 two coins. 



I have got many persons to make this experiment, and in every 

 case with the same results. 



From a knowledge of this fact, I ventured upon the prediction 

 that if two discs of colour exactly complementary to each other 

 could be presented one to each eye, the resulting image seen by the 

 observer would be white. 



Mr. Stevenson, a member of the Council of the Royal Micro- 

 scopical Society, some time afterwards produced such discs by 

 means of polarised light, and the result was as I had predicted. 



John Browning. 



[847] — I marvel somewhat that when correcting a mistake made 

 by " Single Vision," in his letter in Knowledge No. 84, p. 348, 

 you did not also correct the very great mistake in the last sentence 

 of his letter. That sentence, being followed immediately by your 

 note, is virtually endorsed by your silence respecting it, and will 

 mislead dozens of those readers of Knowledge who are as un- 

 acquainted with the elements of optics as " Single Vision." 



Single Vision, No. 2. 



[" No. 2" is quite right ; the mistake should have been corrected. 

 Double vision does result, but not because of unequal magnifying, 

 yet the magnifying is unequal. — E. P.] 



li 



RATIONAL DRESS. 

 [848] — I have read with interest all that has been said in your 

 paper about woman's dress, and have been induced by it to try the 

 new style. I am delighted with the result, and would on no acco mt 

 go back to petticoats and stays. As th';re seems to be a variety of 

 opinion as to what one ought to wear, I will give full details of my 

 costume: flannel combination without band; divided skirt; thiii 

 linen body, to the waist of which is buttoned the dress skirt ; dress 

 body ; stocking suspended from the broad, close-fitting belt of the 

 divided skirt— which belt I consider essential to the comfort of any 

 one leaving off stays for the first time, as it gives snch a feeling of 

 support to the body. I wear the same fashionable dresses as I have 

 been accustomed to (frilled, furbolowed, and steeled), and only one 



lady (and I told her) knows that 1 have altered my undergarments. 

 The keenest feminine eyes have noticed only the greater ease with 

 which I take long walks, and my healthier appearance. I am 

 twenty-two years old and vain enough not to wish to make an 

 object of myself. I assure all women they can try the new dress 

 for themselves >vithout any one being the wiser. 



PEDE.STBIENNE. 

 [Wc are nmch obliged to " Pedestrienne " for her letter. " A 

 Lady," to whom we have shown it, says it agrees with her experience 

 in all respccts.^ — R. P.] 



A FEW OBSERVATIONS ON HEAT. 



[849] — Mr. Davidson says no attempt has ever been made to 

 explain the nature of the product resulting from multiplying si)ccific 

 heat by temperature, so far as he knows. In the " Transactions 

 Roy. Soc. Edin., XX.," the late Professor Rankine clearly explained 

 this thirty years ago. The product is not " momentum," but 

 " energy," and is proportional, not to the velocity, but to the 

 square of the velocity. Mr. Davidson should also have written 

 "absolute temperature" for "temperature." He talks of 

 the momentum of a " single molecule," whereas the 

 statistical method introduced by Clausius and Maxwell should 

 have been adopted, and the " mean square " velocity ascertained. 

 He talks of the " method of mixture." Does he mean the method 

 of mixture of solid substances ? He says the specific heat of water 

 is exactly twice that of the vapour. But he takes no note that the 

 latter varies with temperature and pressure. And at 212° Fah., 

 and 147 lb. per square inch, the specific heat of steam to water is 

 as '37 to 1 — not double. See McFarlane Grav, in Phil. Mag., May, 

 1882. 



In the concluding paragraph, Mr. Davidson says temperature 

 seems to correspond to space; whereas Rankine, Clausuis, Thornton, 

 Tait, and Maxwell, show that it is kinetic energy, not that it corre- 

 spojids to it. Mr. Davidson concludes by explaining the reflection 

 of light by the exploded corpuscular theory '. T. J. Dewar. 



CONSANGUINEOUS M.\RRIAGES. 

 [850] — In reading your report of Dr. Cameron's paper on the 

 above subject in its relation to deaf-mutism, I was reminded that I 

 had lately seen a number of cats and kittens which were nearly all 

 quite deaf. In rearing the animals in qnestion no care was taken 

 to avoid in-and-in breeding; in fact, to keep the breed pure, quite 

 an op])osite plan was adopted, and they are all descendants of one 

 pair obtained some twenty or thirty years ago. The result bears 

 out in a remarkable degree the conclusion arrived at by Dr. 

 Cameron in the paper referred to. R. S. T. 



AN ODD MISPRINT. 

 [851] — Your item in Knowledge for April 27, on the blunders 

 of compositors, recalls an experience of my ovm. some two years 

 since. My right arm was temporarilj' disabled. I commonly an- 

 nounce the topic of my Sunday discourses in our daily newspaper 

 of Saturday. Being unable to write, I stepped into the business 

 office of the paper, and asked a clerk to write a notice for me, 

 saying that my subject on Sunday morning would be " Caesar's 

 Things, and God's." Judge of my surprise on reading, a few hours 

 later, that I would speak on " Corsairs, Thugs, and Gods ! " And 

 I abhor sensationalism in the pulpit ! W. G. H.i.skell. 



LANDRAILS. 

 [852] — Every observant country rambler cannot fail to have 

 noticed the unusual numbers of landrails (Crer pratensis) that are 

 to be met with. The voice of this meadow bird, softened by dis- 

 tance, is one of the most characteristic of our rural charms ; but 

 when one's ears are assailed on all sides with the monotonous cry of 

 this bird, the usual pleasant effect is assuredly marred. From dif- 

 ferent parts of our island I have heard of their phenomenal abun- 

 dance ; indeed, they seem to be a sort of aftliction this year — they 

 literally and positively swarm. Did we live in the times and countiy 

 of Moses and Phar.aoh, I should be decidedly inclined toattribute their 

 numbers to an eleventh plague. Out in the fields one hears nanght 

 but their abominable crake-crake, crake-crake, and towards dusk 

 their persistency is something marvellous. The very air seems 

 saturated with their outrageous clamouring, and the otherwise 

 poetic gloaming becomes very prosy indeed. I am here just half-a- 

 mile from the centre of the old Danish burgh of Derby, and yet, 

 with my window open, 1 can distintly hear the subject of my letter 

 rasping out his harsh song, as if his very life depended on the 

 effort. To my knowledge, this is the first time for years that 

 they have approached the town so near. There is something 

 essentially romantic in the idea of being serenaded, on a moon- 

 light night, beneath one's chamber window ; but when the 

 "gay troubadour" turns out to be a saw-voiced, crack-throated 



