204 THE POPULAR SCIENCE MONTHLY. 



our present instruments we can perceive lines ruled on glass of 

 ^J-ij-u of an inch apart. But, owing to the properties of light 

 itself, the fringes due to interference begin to produce confusion at 

 distances of - l4: l 00 , and in the brightest part of the spectrum at little 

 more than -t r o ^ 00 they would make the obscurity more or less com- 

 plete. If, indeed, we could use the blue rays by themselves, their 

 waves being much shorter, the limit of possible visibility might be 

 extended to x 2 t/o 5 anc ^> as Helmholtz has suggested, this perhaps 

 accounts for Stinde having actually been able to obtain a photographic 

 image of lines only 1 u u \ u of an inch apart. It would seem, then, 

 that, owing to the physical characters of light, Ave can, as Sorby has 

 pointed out, scarcely hope for any great improvement so far as the 

 mere visibility of structure is concerned, though in other respects, no 

 doubt, much may be hoped for. At the same time, Dallinger and 

 Royston Pigott have shown that, so far as the mere presence of sim- 

 ple objects is concerned, bodies of even smaller dimensions can be per- 

 ceived. 



Sorby is of opinion that in a length of 8 * of an inch there 

 would probably be from 500 to 2,000 molecules 500, for instance, in 

 albumen and 2,000 in water. Even, then, if we could construct micro- 

 scopes far more powerful than any we now possess, they would not 

 enable us to obtain by direct vision any idea of the ultimate molecules 

 of matter. Sorby calculates that the smallest sphere of organic matter 

 which could be clearly defined with our most powerful microscopes 

 would contain many millions of molecules of albumen and water, and 

 it follows that there may be an almost infinite number of structural 

 characters in organic tissues, which we can at present foresee no mode 

 of examining. 



The science of meteorology has made great progress ; the weather, 

 which was formerly treated as a local phenomenon, being now shown 

 to form part of a vast system of mutually dependent cyclonic and 

 anti-cyclonic movements. The storm-signals issued at our ports are 

 very valuable to sailors, while the small weather-maps, for which we 

 are mainly indebted to Francis Galton, and the forecasts, which any one 

 can obtain on application, either personally or by telegraph, at the 

 Meteorological Office, are also of increasing utility. 



Electricity, in the year 1831, may be considered to have just been 

 ripe for its adaptation to practical purposes ; it was but a few years 

 previously, in 1819, that Oersted had discovered the deflective action 

 of the current on the magnetic needle, that Ampere had laid the foun- 

 dation of electro-dynamics, that Schweitzer had devised the electric 

 coil or multiplier, and that Sturgeon had constructed the first electro- 

 magnet. It was in 1831 that Faraday, the prince of pure experimen- 

 talists, announced his discoveries of voltaic induction and magneto- 

 electricity, which, with the other three discoveries, constitute the 

 principles of nearly all the telegraph instruments now in use ; and in 



