We are but beginning to realize the marvelous range and complexity of Nature. I have elsewhere called attention to this with special reference to the problematical organs of sense possessed by many animals. [2] There is every reason to hope that future studies will throw much light on these interesting structures. We may, no doubt, expect much from the improvement in our microscopes, the use of new re-agents, and of mechanical appliances; but the ultimate atoms of which matter is composed are so infinitesimally minute, that it is difficult to foresee any manner in which we may hope for a final solution of these problems. Loschmidt, who has since been confirmed by Stoney and Sir W. Thomson, calculates that each of the ultimate atoms of matter is at most 1/50000000 of an inch in diameter. Under these circumstances we cannot, it would seem, hope at present for any great increase of our knowledge of atoms by improvements in the microscope. With our present instruments we can perceive lines ruled on glass which are 1/90000 of an inch apart; but owing to the properties of light itself, it would appear that we cannot hope to be able to perceive objects which are much less than 1/100000 of an inch in diameter. Our microscopes may, no doubt, be improved, but the limitation lies not in the imperfection of our optical appliances, but in the nature of light itself. It has been calculated that a particle of albumen 1/80000 of an inch in diameter contains no less than 125,000,000 of molecules. In a simpler compound the number would be much greater; in water, for instance, no less than 8,000,000,000. Even then, if we could construct microscopes far more powerful than any which we now possess, they could not enable us to obtain by direct vision any idea of the ultimate organization of matter. The smallest sphere of organic matter which could be clearly defined with our most powerful microscopes may be, in reality, very complex; may be built up of many millions of molecules, 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. [3] Again, it has been shown that animals hear sounds which are beyond the range of our hearing, and I have proved they can perceive the ultra-violet rays, which are invisible to our eyes. [4] 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 animals 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 creatures would differ from our white light in containing this additional color.