BEYOND THE LIMITS OF VISION. 



bers than the universe of the heavens. We will follow it grad- 

 ually down until it vanishes from all possibility of conception. 



The limit of natural vision I suppose is not far from the one- 

 hundredth of an inch. The volvox, which can be seen by the 

 naked eye as a little green speck in pond water, is about the one- 

 fiftieth of an inch in diameter. The largest diatoms, which are 

 seen as the merest particles of fine dust, seldom reach the size 

 of one-hundredth of an inch. The finest point of the sharpest 

 needle is estimated at the one-thousandth of an inch across, and 

 as you know can no more be seen than the edge of a razor. You 

 can of course tell where it is, for it stops the passage of light ; 

 but you can see no dimensions about it whatever. 



With the microscope the limit of .resolving power is somewhere 

 near the hundred-thousandth of an inch. The test rulings, known 

 as Robert's bands, are lines made by the point of a diamond on 

 glass guided by the finest screws and machinery. The nineteenth 

 band is ruled to the known fineness of 120,000 to the inch ; and 

 notwithstanding some claims to the contrary, I think it exceed- 

 ingly doubtful if these lines have ever been clearly separated by 

 the highest powers of the microscope. 



One of the most interesting and complete illustrations of the 

 use of the highest powers of the microscope that I have ever 

 read, is in a monograph on one of the infusoria, by the Rev. Dr. 

 Dallinger, a distinguished English microscopist. He has followed 

 this animalcule, the greatest length of which is only the one ten- 

 thousandth of an inch, through all the phases of its life history, 

 comprised within ten to twelve hours. A full grown individual 

 divides itself lengthwise into two perfect beings in about five 

 minutes. In another five minutes, each of these go through the 

 same operation again, and so on for hours. After from three to 

 seven hours of this kind of multiplication, the older ones die off, 

 while some of the younger and more vigorous attach them- 

 selves to each other in pairs. One entirely absorbs the body 

 of the other into its own, and settles down to the quiet cysted 

 state, as it is called. Then after a certain time there commence 

 to ooze out of this body perfect little clouds of the minutest 

 spores, until nothing is left of the parent organism but the 



