26 THE MICROSCOPE. 



most important part in the formation of microscopical 

 images, since dissimilar structures give identical images 

 when the difference of their diffractive effect is re- 

 moved, and conversely similar structures may give dis- 

 similar images when their diffractive images are made 

 dissimilar. Whilst a purely dioptric image answers 

 point for point to the object on the stage, and enables 

 a safe inference to be drawn as to the actual nature 

 of that object, the visible indications of minute struc- 

 ture in a microscopical image are not always or neces- 

 sarily conformable to the real nature of the object 

 examined, so that nothing more can safely be inferred 

 from the image as presented to the eye, than the 

 presence in the object of such structural peculiarities 

 as will produce the particular diffraction phenomena 

 on which these images depend. 



It should be carefully noted that diffraction is not 

 limited to lined objects, it applies to structures of all 

 kinds. But lined objects give brighter and more dis- 

 tinct diffraction spectra, and are best suited for experi- 

 mental illustration. Nor, again, is diffraction limited to 

 transparent or semi-transparent objects viewed by trans- 

 mitted light. It equally applies to opaque objects, and 

 is, in fact, universal whenever the strictly uniform 

 propagation of the luminous waves is disturbed by the 

 interposition either of opaque or semi-opaque elements, 

 or of transparent elements of unequal refraction, which 

 give rise to unequal retardations of the waves. 



The Simple Microscope. 



A single lens, or a sphere of glass or water, forms a 

 simple microscope, or, as it is more familiarly called, a 

 magnifying glass. Lenses are ground of various forms, 

 as represented in fig. 13 ; a is a plane glass of equal 

 thickness throughout ; &, a meniscus, concave on one 

 side, convex on the other ; c, a double-concave : d, a 

 plano-concave ; e, a double-convex ; /, a plano-convex. 



By a proper combination of certain forms of lenses, 

 we unite on the same sensible point a number of rays, 

 proceeding from the same point of an object, each 



