296 optics. 



of the mite is T lSs part of an inch, which is almost 

 one-tenth part of an inch. 



Again, suppose you measure the length of the 

 egg of a mite, and find one turn of the screw, 

 and three divisions on the plate, carry it completely 

 under the wire; then one revolution of the screw 

 is-rV or t^57, and three divisions are tsW; so the whole 

 length of the egg is equal to tH* of an inch, that 

 is, almost one forty-fourth part of an inch; or 

 forty-four such eggs of a mite will, if laid con- 

 tiguously in a right line, be nearly equal to one 

 inch in length. 



The micrometer may be easily applied to the 

 solar microscope; for let a fine straight line be 

 drawn on the screen, and the end or side of the 

 image be placed to touch it ; then by turning the 

 screw, it will thereby be measured in thousandth 

 parts of an inch. 



By the micrometer the naturalist acquires an 

 idea of the wonderful disproportion there is be- 

 tween the magnitudes or dimensions of the small- 

 est and of the largest objects of the animal tribe; 

 as between the smallest of the finny fry and the 

 largest whale ; an animalcule in vinegar, and a 

 snake thirty or forty feet long. And that the 

 reader may have a perception of such a surprizing 

 contrast in the works of Nature, we shall here 

 give him a calculation of the comparative magni- 

 tude of the egg of a mite, and that of an ostrich. 



Suppose the length of an ostrich's egg to be 

 five inches, let the length of the egg of a mite be 

 one-fiftieth of an inch, then the lengths of these 

 two eggs will be to each other as five to one-fiftieth, 

 or two hundred and fifty to one; then, as they are 

 similar bodies, their magnitudes will be as the 

 cubes of these numbers, viz. as 15,625,000 to one; 



