206 MICROSCOPICAL IMAGERY. 



When obliquity is established, rare and most beautiful forms 

 arise, resembling parachutes, vases, or comets, made up of ellip- 

 soid, parabolic, or hyperbolic diffraction lines. These effects 

 depend on the aberrations present and the arrangements of the 

 optical axes. Inaccurate centreing of the objective lenses is at 

 once detected by the production of excentric patterns and more 

 than one central disc ; whilst bad curvatures of the glasses pro- 

 duce irregular shapes in the central black rings, and sometimes a 

 variety of spurious discs. At other times, with inferior glasses, 

 the beauty of the rings is entirely marred, very few can be deve- 

 loped, and no black rings at all can be seen.* 



These researches point to many difficulties to be encountered 

 under high-power definition, especially in the observation of 

 brilliant organic particles. Thus the molecules seen in the cele- 

 brated Podiira scales baffle almost all observers. Using a fine 

 I — 50th immersion by Powell and Lealand (price 3c guineas ; 

 this glass is now charged ;^8o apochromatic) without any obliquity 

 of illumination, but employing a tube shortened four inches and a 

 B eyepiece (power, 1330), the upper and continuous ribs of the 

 Podiira were resolved into strings of blue sapphire-like beads 

 appearing perfectly circular. The interspaces between the mark- 

 ings, at a lower focus^ showed subjacent strings of white beads. 

 Monads appeared blue, swimming about in the water-immersion 

 used, and also lying in well-defined masses. Some of them could 

 be seen to rotate. The cilia were invisible, but the movements 

 gave strong indications of them. A blue glass improved the 

 definition (petroleum-oil lamp, ij-inch objective for condenser, 

 without stops centrally used) 



At present, nothing is more difficult of definition in the 

 microscope than an assemblage of minute, refracting, organic 

 particles. Virtually forming discs of light, these evolve the dif- 

 fraction-errors and phenomena more or less vividly. No English 

 microscopist, so far as is known to the writer, has succeeded in 

 displaying these beautiful beaded structures existing between the 

 celebrated exclamation markings of the Podura test-object. Here 

 closely-packed masses of organic particles, highly refractive, trans- 

 parent, and diffiactive, obscure each other; brilliant points are 

 * Yox a fuller account, see Proc, Royal Society^ No. 146, 1873. 



