AND THE MICROSCOPE. 31 



indeed most frequently, only nominal, like the magnifying 

 millions of times by the oxy-hydrogen microscope which 

 the charlatans boast of, and which in most instances does 

 not exhibit so much as the fifty times of a good common 

 microscope. 



From these observations it will be evident that to the 

 scientific inquirer it is of infinite importance to be able to 

 form an accurate judgement of the goodness of an instru- 

 ment in this respect, and the greatest efforts have been 

 used to discover some means thereto. To this end, have 

 been sought what are called test-objects, which generally 

 consist of objects exhibiting some delicate structure, diffi- 

 cult to make out. Either artificial or natural objects may 

 be chosen for such test-objects. The former have only 

 been prepared hitherto by Robert, an optician in Konigs- 

 berg, and consist of glass plates on which are ruled with a 

 diamond, systems of a hundred lines which, ten by ten, 

 approach closer together and are finer, according to a 

 definite standard. With most instruments only the sixth 

 and seventh systems can be distinctly made out to be 

 composed of separate lines, superior instruments reach the 

 eighth and ninth. No instrument yet constructed has 

 resolved the tenth system into its component parts. These 

 systems of lines when they are perceived, are very evident, 

 but they have the essential fault that they do not quite 

 exactly resemble one another, and consequently every 

 inquirer gets a different standard. With incomparably 

 greater accuracy works nature, and therefore the scales of 

 butterflies are always regarded as the best test-objects. 

 These are mostly little longish plates having a little stem, 

 beset upon their surface with fine longitudinal furrows, 

 which are united by the most delicate possible transverse 

 striae. These two kinds of striae are of very different 



