Feb. 10, 1882.] 



KNOWLEDGE 



311 



AN ILLliSIRATED ^^ i^ 



MAGAZINEoFSqENCE ^ 

 1- plainltWoriied-Exact&described^ 



LONDON: FRIDAY, FEBRUARY 10, 1882. 



CoSTF.N'TS OF No. l-" 



I>AOK 



The r.ve imd the ^licro'soope. Hv 



Henrv J.Slack, F.O.S.. K.R.M.S. 311 

 About Fallacies. By the Kdilor ... 3U 

 Niehta with a Three-Ineh Tele^oop.?. 



Br " A Fellow of the H.jral A«!ro- 



nonilcal Societv." {Jlliulr,iled\ . 312 

 The Electric telegraph. By W. 



Lynii 313 



The Croat PiTamid. By the Editor 



(llUstrale'd) 313 



The Crystal Palaee Electrical Ethi- 



bilion. First Xoticc 319 



Natural Rubbish Heaps. By .lames 



Geikie, LL.D., F.R.S 31S 



Dr. Carpenter 0» Vaccination ..... .. 319 



The Moon's Birth by Tidal Evolu- 

 tion 310 



The Menacing Com-t 320 



CoRBBSPOlfDRTfCB ; — Our Letters, 

 Queries. !ind Replies— Flesh Food 

 — The Mi>i>n and the Weather — 

 Intra-Mereurial Planet, &c. ...321-32,3 



Queries 323 



Replies to Queries 321 



.\n3wer8 to Correspondenta 3i.T 



Letters R.-oeived 327 



Notes on Art and Science 327 



Our Mathematical Column 32S 



Our Whist Column 321 



Our Chess Column 330 



THE EYE AND THE MICROSCOPE. 



By HENRr J. Sl.\ck, F.G.S., F.R.M.S. 



THERE is a notion prevalent that using the microscope 

 injures the eyesiglit, but this is really not the case ; 

 it is only the abuse of the instrument that has such an 

 effect. It does not hurt the eye to look at anything which 

 is plain and easy to see, and neither in too strong nor too 

 feeble a light. The art, then, of u.sing the microscope con- 

 sists in exhibiting small objects so that tliej- shall be seen 

 as larger ones are witliout any instrumental aid. To do 

 this the microscope must bo a good one, and a few simple 

 rules followed until practical skill is gained. Whether the 

 microscope is a binocular or not, both eyes must lie kept 

 open. This is of absolute necessity if the sight is not to 

 be damaged. Many persons find it difficult to look down a 

 tube with one eye without shutting the other. A remedy 

 for this, and one useful in other respects with a mono- 

 cular instrument, is to cover a piece of cardboard nine 

 inches long and five wide, with black cotton velvet, 

 cut a hole in the middle, and insert the eye - piece 

 through it into the tube of the microscope. With this 

 screen in front, everybody finds it easy to keep both 

 eyes open, and look at the object with either one, if they 

 arc both .alike, or with the best if they diflTor. Persons 

 whose eyes agree in focus, and othci-wise correspond, derive 

 the greatest benefit from binocular instruments, but the 

 use of one eye does not fatigue it if the object is properly 

 focussed and suitaljly illuminated. A common fault with 

 beginners is to use too high power, with which it is im- 

 imssiblo to see the object tliey want. For many most 

 interesting sights of live objects a four-inch objective is 

 extremely useful, and the highest power a beginner is 

 likely to employ with advantage is a one-inch, with a 

 couple of eye-pieces, the highest giving a linear magnifica- 

 tion of ."lO or 60, with the English length of tube. Tlie 

 proper use of this power should be mastered before more 

 magnification is attempted. 



It requires considerable practice to pay attention only to 

 one part of an object that may be shown plainly, and take 



no notice of other parts that from any reason are not clear. 

 With low powers, an object need not be (juite flat for the 

 whole to be fairly in focus at the same time ; with higher 

 powers, great flatness is indispensalile,"and an extremely 

 slight irregularity only porinits one portion at a time to be 

 clearl)' seen. This state of things is very uncomfortable to 

 a beginner, and the eye is sure to suflor from it. The size 

 of an object that can be viewed as a whole with any power 

 depends upon several conditions, which need not now be 

 discussed, l)ut the higher the magnification the loss it is, 

 and with an enlargement exceeding 200 linear it becomes 

 exceedingly minute. 



As soon as the student has attained to some dexterity in 

 the use of the in.strument, he should acquire the habit of 

 paying exclusive attention to what he can see plaiidy, and 

 take no notice whatever of things out of focus, or from any 

 other cause not even focally visible. With some excep- 

 tions, it is most .agreeable to the eye that an object should 

 not occupy the whole field, but have a fair margin round 

 it, which should not be over-flooded with light. Eyes differ 

 very much in sensitiveness to light, and when, as in no- 

 ticing the actions of live objects, prolonged .attention is re- 

 quired, the intensity of the illumination .should be nicely 

 regulated to suit the individual case. Light passing through 

 a piece of foreign post-paper, saturated with spermaceti, is 

 often the most pleasant. 



By attention to such directions as have been mentioned, 

 there need be no fear of devoting a considerable time every 

 day to microscopic investigation, and many obser\'ers could 

 be mentioned who have done this for years without any 

 detriment to their visual powers. On the other hand, those 

 who torment their eyes in attempts to see the most difficult 

 diatom markings, or the closest of Nobert's ruled lines, suffer 

 from their folly, without any compensation in the shape of 

 useful knowledge. 



ABOUT FALLACIES. 



By the Editor. 



SEVERAL correspondents write about the question of 

 luck as wc considered it in No. 11, .some asking 

 whether the evidence does not show that some men really 

 are exceptionally lucky (so that their luck in matters of 

 pure cliance may be relied on); others asking whether, if a 

 coin had been tossed a great number of times with the 

 same result (head or tail) in one sot of trials, it would not 

 be more likely to show the other side (tail or head) oftener 

 in the next set of trials ; while yet others consider that the 

 ideas of men of science about fallacies generally are 

 erroneous — that, in fact, the so-called fallacies are real 

 truths. 



Taking the last first, I may note that the rule of science 

 in all those eases in which specific results arc popularly 

 supposed to followfrom specified actions, or the like, is simply 

 to inquire whether there can possibly be any relation of cause 

 and effect in such cases. When a housemaid says, for instance, 

 that putting the poker acro.ss a fire makes the tire burn up, 

 the student of physical laws is able at once to see that the 

 supposed influence is antecedently most improbable. Here 

 in a grate are certain more or less combustible materials, 

 and certain qu.antities of m.atter already burning; com- 

 bustion is going on, though indiff'creiitly ; the air is 

 nourishing this slowly burning fire, but inetllciently ; on 

 the whole, it seems likely that the fire will go out. In 

 what w.ay shall I do any good if I stick a rod of iron from 

 the fender across the top bar? I thus add a certain 

 qu.antity of cold metal to the space across which the air 

 has to come to the fire. Do I increase the draught 1 On 



