MICROSCOPIC OBJECTS. 



39 



have therefore been denominated tests, by their discoverer Dr. Goring. 

 In order that the reader may be able justly to appreciate the efficiency 

 of any microscope that may come within his observation, and deter 

 mine its penetrating and defining powers, whether the instrument be 

 single or compound, I shall describe the principal test objects neces- 

 sary for that purpose. The objects best adapted to determine the 

 penetrating power are the dust or scales from the wings of certain 

 classes of papilio, (butterflies and moths.) Of these, the menelaiit, 

 shown in figures 21 and 25 of the following illustration, is a very useful 

 object. The dust from the under side of the wing of the male papilio 

 brassica, (white cabbage butterfly,) shown in fig. 22, is a good proof 

 object ; and a very peculiar one of the same kind is shown in fig. 24, 

 (both magnified.) In viewing these objects, a large angle of aperture 

 is required, (at least equal to half the focus,) in order that the lines 

 and markings may be distinctly seen. 



Fig 21. Fig. 22. Fig. 23. 



Fig. 24. Fig. 25. 



There are, however, many of the scales from some kinds of papilio on 

 any of which the lines can be seen by an ordinary instrument. But 

 the objects here selected, as well as the lines on the scales of the 

 small brown house moth ; the lines on the scales, taken from the 

 foreign curculio, (diamond beetle,) fig. 23, require a more perfect 

 instrument to develope them. 



Mr. Pritchard recommends for this purpose, the scales from the 

 wing of the Euploea limniace, and the blue ones from the Papilio 

 Paris as valuable objects, as the cross striae on them are strongly and 

 easily developed under a power of from 100 to 200 times linear. 



Lastly The most difficult of all the test objects, are the lines on 

 c 2 



