--Mr. Sorby has lately made some calculations of the probable size of the invisible atoms which compose material substances. Dr. Royston Pigott determined that the smallest visual angle which we can well appreciate is that covering a hole of 11.4 inches diameter at a distance of 1,100 yards. This corresponds to about six seconds of an arc. In a microscope magnifying 1,000 diameters this would make visible a particle one-three-millionth part of an inch thick. But Mr. Sorby is inclined to think that a size between 1/80,000 and 1/100,000 of an inch is about the limit of the visibility of minute objects, even with the best microscopes. Now, taking the mean of the calculations made by Stoney, Thomson, and Clerk-Maxwell, we have 21,770 as the number of atoms of any permanent gas required to cover one-thousandth of an inch, when lying end to end. By a series of calculations which produce numbers entirely beyond human conception, (10,317,000,000,000 atoms in 1/100,000,000 of a cubic inch, for instance) he reached the conclusion that there are in the length of 1/80,000 of an inch (the smallest visible object) about 2,000 molecules of water, or 520 of albumen, and therefore, in order to see the ultimate constitution of organic bodies, it would be necessary to use a magnifying power from 500 to 2,000 times greater than those we now possess. With this result settled, he was able to make one of those radical predictions which are so rarely possible to the careful scientist; namely, that the atom will never be seen by man. It is not that instruments cannot be made powerful enough (though that is no doubt true), but that the waves of light are too coarse to distinguish the limits of such an extremely small distance. To see atoms we should need light waves only one-two-thousandth of their actual length. At present we are as far from that attainment as we are from reading a newspaper, with the naked eye, at the distance of one-third of a mile.