Ja>\ 1, IS Go.] 



SCIENCE-GOSSIP. 



and see ' distinctly that -whicli is submitted to 

 it. But we have eveiy reason to suppose 

 that if the eye could be brought within an 

 inch of a small object, and see it clearly, it 

 would appear much larger than it does to 

 the unassisted eye at the ordinary distance. 



The balloon and the castle as they came 

 nearer to the spectator kept increasing in 

 size, according to a well-known law, and 

 would have continued to do so had they not 

 been stopped by the inability of the eye to 

 see them distinctly at a lesser distance than 

 its proper focus. There are, however, strong- 

 sighted persons, whose eyes, having a much 

 shorter focus than their neighbours, are able 

 to look more closely into some matters than 

 their less-gifted brethren. Such may be seen 

 readiug a favourite author with their noses 

 in contact with the page. Their vision is, in 

 fact, microscopic. They see everything on a 

 larger scale than ordinary folks. Try them 

 witli the minutest specimens of caligraphy or 

 printing, and they excite your envy by read- 

 ing with ease that which you can barely 

 decipher by the aid of a lens. Whence this 

 power 1 It will be found, if we mistake not, 

 in the difference of the organization of such 

 an eye. Our friend the physiologist here 

 comes to our aid : by the light he has thrown 

 upon the subject we perceive that the cornea 

 of the short-sighted eye has a deeper curve 

 than the cornea of the ordinary eye, and this 

 may also apply to the crystalline lens. Such a 

 departure from the normal form tends greatly 

 to increase the refractive power of the eye, 

 and thus to shorten its focus. But whilst 

 an eye of this description has an advantage 

 in being able to see objects distinctly at a 

 short distance, and consequently much mag- 

 nified, it labours under the great disadvantage 

 in being unable to distinguish a friend from 

 a lamp-post on the opposite side of the sti'eet. 

 Without the aid of concave glasses to 

 elongate his vision it would be impossible for 

 a short-sighted person to realize the poet's 

 expression — - 



'Tis distance lends enchautmcut to the view. 



As we who possess ordinary eyes cannot 

 alter their natural organization, it is pretty 

 clear, if we wish to increase their power, that 

 we must seek for the material wherewith to 

 do so in the region of ojjtics. 



Ordinary objects are seen by th.e light they 

 reflect from all points of their surfaces. 

 Thousands of such rays emanate from the 

 objects that ai'e round about us, each ray 

 bearing the form and colour of that particular 

 spot from v/hence it has proceeded. These 



enter the eye through that small hole — the 

 pupil — and, in the aggregate, form on its retina 

 a i^erfect miniature picture. Such, however, 

 is the result only when the eye is at a proper 

 distance from an object ; when it approaches 

 too closely it fails to perceive it. There is no 

 distinct vision, owing to the light's rays di- 

 verging at too great an angle. Many of them 

 do not enter the eye at all, but shoot by it ; 

 while those raj's that do gain an entrance are 

 still too divergent for the eye to bring them 

 into a proper condition for forming a perfect 

 image on the retina. Now as the mind can 

 receive no true idea of the form and colour of 

 external objects except through the instru- 

 mentality of the eye, it will necessarily follow 

 that distinct vision depends on the accuracy 

 of the picture formed on its retina. Where 

 the image produced is hazy and indistinct, the 

 mental impression will be equally obscure. 

 Knowledge is said to be power. It certainly 

 is so when applied to optics. By our re- 

 searches we become acquainted with the fact 

 that when a ray of light, in an oblique or 

 slanting direction, enters a transparent me- 

 dium denser than that through which it has 

 pi'eviously travelled (such as water or glass), 

 it suffers refraction ; that is, it is bent out of 

 its original course. Taking advantage of this 

 fact, we procure a piece of clear glass, give to 

 each of its surfaces a curve like that described 

 by a bent bow, and then highly polish them. 

 This is termed a convex lens, and when 

 mounted on an arm, or in some kind of handle, 

 it becomes a hand-magnifier', or simple micro- 

 scope. The action of such a lens on rays 

 diverging from a brilliant point (if that point 

 be in its focus) is to render them parallel, or 

 if the point be nearer to the lens than its true 

 focus, the divergentrays will be rendered lessso. 

 If parallel rays be transmitted they will come 

 to a point at some distance beyond the lens. 

 This is its true focus for parallel rays, and 

 any small object placed here and properly 

 illuminated would be more distinctly seen than 

 in any other position. By giving to our 

 piece of glass a deeper curve, or by selecting 

 a denser medium than glass, such as a 

 diamond or ruby, we obtain a lens still more 

 powerful in its refractive properties. We 

 have thus the means of providing ourselves 

 with lenses of longer or shorter foci, and of 

 different degrees of magnifying power. All 

 lenses of high power have short foci, and 

 must be held very close to an object in order 

 to see it ; in some instances almost in contact 

 with it. Now observe how cunningly that 

 animal, man, takes advantage of his own con- 

 trivance. He has by his skill constrLiCted au 



