io6 THE POPULAR SCIENCE MONTHLY. 



is at most yo-sTrVrmr of an inch, in diameter. Under these circum- 

 stances we can not, it would seem, hope at present for any great 

 increase of our knowledge of atoms by improvements in the mi- 

 croscope. With our present instruments we can perceive lines 

 ruled on glass which are ^o^^-q of an inch apart. But, owing to 

 the properties of light itself, the fringes due to interference begin 

 to produce confusion at distances of *ii\qq , and in the brightest 

 part of the spectrum, at little more than 90^00 , they would make 

 the obscurity more or less complete. If, indeed, we could use the 

 blue rays by themselves, their waves being much shorter, the limit 

 of possible visibility might be extended to tts^ot ; and, as Helm- 

 holtz has suggested, this perhaps accounts for Stinde having 

 actually been able to obtain a photographic image of lines only 

 ro^roT of an inch apart. This, however, would appear to be the 

 limit, and it would seem, then, that, owing to the physical char- 

 acters of light, we can 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, as far as the 

 mere presence of simple objects is concerned, bodies of even small- 

 er dimensions can be perceived. According to the views of Helm- 

 holtz, the smallest particle that could be distinctly defined, when 

 associated with others, is about -g-gwo of an inch in diameter. 

 Now, it has been estimated that a particle of albumen of this size 

 contains 125,000,000 molecules. In the case of such a simple com- 

 pound as water, the number would be no less than 8,000,000,000. 

 Even then, if we could construct microscopes far more powerful 

 than any we now possess, they could not enable us to obtain by 

 direct vision any idea of the ultimate molecules of matter. The 

 smallest sphere of organic matter which could be clearly defined 

 with our most powerful microscopes may be, in reality, very com- 

 plex ; may be built up of many millions of molecules, and it fol- 

 lows that there may be an almost infinite number of structural 

 characters in organic tissues which we can at present foresee no 

 mode of examining. 



Again, it has been shown that animals hear sounds which are 

 beyond the range of our hearing, and that they can perceive the 

 ultra-violet rays which, are invisible to our eyes.* 



Now, as every ray of homogeneous light which we can perceive 

 at all appears to us as a distinct color, it becomes probable that 

 these ultra-violet rays must make themselves apparent to the ants 

 as a distinct and separate color (of which we can form no idea), 

 but as different from the rest as red is from yellow, or green from 

 violet. The question also arises whether white light to these 

 insects would differ from our white light in containing this addi- 

 * " Ants, Bees, and Wasps." 



