THE MICROSCOPE AND ITS PREPARATIONS. 165 



answer ; and so far as I know, I have just been asking precisely 

 such questions. 



There is a lively little insect, the largest of which is not more 

 than the tenth of an inch long, called the Spring-tail or Podnra, 

 found in damp saw-dust in wine cellars, or in any moist and pro- 

 tected places near decaying wood. You may catch or collect 

 them by sprinkling a little flour on a piece of paper and placing 

 it near their resorts. The body of this insect is covered with 

 minute scales, remarkable for their almost infinitesimal markings. 

 The finest line that can be made, visible to the unaided eye, is 

 about the 250th of an inch in breadth. The rulings on writing 

 paper are at least five times as broad, or about the 50th of an 

 inch in width. Now across that finest line, which you can only 

 see as a mere mark without breadth at all, as many as four or five 

 of these Podura scales could be placed side by side. And across 

 each scale in waving lines are rows of forty or fifty dashes, like 

 exclamation points, side by side. This makes them, as you may 

 readily figure, about 50,000 to the inch. This Podura scale used 

 to be the test for the highest powers of the microscope ; but now 

 object glasses are made of such perfection that test objects 

 of double that number of lines to the inch are required. These 

 are found in the shells of a certain diatom, the Amphipleura 

 pellucida, found quite common in almost any pool or rivulet. 

 The rulings or striae running across the frustules of this diatom 

 are as fine as anything that has as yet been found in nature, ex- 

 ceeding in some cases 100,000 to the inch. Yet Dr. Woodward, 

 of Washington, has repeatedly photographed portions of this 

 shell, showing distinctly and clearly markings so fine that 400 of 

 them might be drawn lengthwise on the finest line that you can 

 see with the naked eye. 



But I did not come before you to-night to speak of the most 

 difficult of the objects which this instrument resolves ; but 

 rather of the beautiful and interesting preparations, requiring 

 not nearly so high a power and satisfying something more than 

 curiosity. I am myself inclined to search for the attractive and 

 the beautiful for exhibition with the microscope ; and whenever 

 I find anything that satisfies in those respects, I am very apt to 



