MICROSCOPIC OBJECTS. 103 



properties in a microscope, and that the instrument might 

 possess a very considerable approximation to perfection 

 in the one, and fall short in the other, or vice versa, or 

 might be perfect in both. The lines on the dust or 

 feathers from the wings of the Lepidoptera, and those on 

 the scales from the body and limbs of the Thysanurseous 

 insects, offered the means of determining their goodness 

 in one particular, namely, their penetration ; while the 

 structure of the hair of animals, certain mosses, &c. 

 served to ascertain their defining power. 



" The analogy between telescopes and microscopes is 

 so great, that I cannot be said to digress from my sub- 

 ject in stating that the aforesaid observations apply also 

 to the former of these instruments, which seldom com- 

 bines the two qualities of penetration and definition to 

 any great extent. Thus a telescope with a large apertm'e 

 wlQ frequently resolve clusters of stars and exliibit nebulce, 

 while it will fail in defining the disc of a planet, or the 

 moon, with precision ; and on the other hand one of 

 moderate aperture accurately figured will define the latter, 

 but be wholly inert on the nebulae and clusters. So a 

 microscope with large aperture and high power will 

 show the * active molecules ' and lined objects, while it 

 will not define a leaf of moss, or a mouse-hair ; and 

 another with a smaller aperture will define the latter, 

 but prove ineffective on the former. This is very ma- 

 nifest in single lenses,* which require different aper- 

 tures for different objects. 



* " I have a very beautiful sapphire lens (plano-convex of one- 



