190 On the Measurement of Minute Particles. [Sept. 



Article V. 



Remarks on the Measurement of Minute Particles, especially/ 

 those of the Blood and of Pus. From Dr. Young's Medical 

 Literature, 8vo. Lond. 1813, p. 545. 



(Concluded from p. 122.) 



V. Microscopical Fallacies. 



I shall here take the liberty of inserting some remarks, which 

 I cannot attempt at present to render intelligible to any, who 

 have not entered into the minutest refinements of physical op- 

 tics ; to such as are unacquainted with the latest investigations, 

 I fear they must appear involved in a degree of obscurity al- 

 most enigmatical. 



When a small object is viewed in a microscope, especially if 

 the light is admitted by a limited aperture, it will often appear 

 to be surrounded by some lines of light and shade, or of colours, 

 which might be supposed to depend on its magnitude, in the 

 same way that the eriometrical colours are derived from the 

 magnitude of the objects examined. In reality, however, their 

 existence and their dimensions depend on the aperture of the 

 microscope, and not on the magnitude of the particles in its 

 focus. To prove that this aperture may produce such an effect, 

 hold any object, for instance, the finger or the nail, so as to 

 intercept all the light of a candle, except a narrow line, and 

 this line will seem to project other lines parallel to it into the 

 adjoining shade. Now these lines depend on the interposed 

 object on one side, and on the margin of the pupil on the other: 

 for if we take an object a little narrower than the pupil, we may 

 see them on both sides of it; and causing the pupil to contract 

 by throwing more light on the opposite eye, they will expand, 

 as the space, through which they are admitted, is diminished by 

 the contraction. We may also very distinctly observe, if we 

 look in this manner at a narrow line of light instead of a candle, 

 that the dispersive powers of the eye manifestly convert its image 

 on the retina into a spectrum of red, green, and blue light : 

 sufficiently confuting the conjectural hypothesis of the achro- 

 matic property of its refractive substances. If again we substi- 

 tute a minute hole or slit in a card for the interposed object, the 

 sides of this aperture will now determine the magnitude of the 

 fringes which are seen at the edge of the candle, and their 

 dimensions will be no longer variable, whatever may be the state 

 of the pupil. But the candle must in this case either be placed 



at a distance, or be partly concealed from the eye, unless one 



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