The London, Edinburgh and Dublin philosophical magazine and journal of science . l be found instructive. 40. Intercostal Markings and allied phenomena. See § 15,p. 345.—Light which does not contribute to delineate any-thing upon the object is apt to intrude in three forms—either (a) concentrated into patterns which are superposedupon the microscopic image, or (b) scattered in patches overparts of it, or (r) spread in the form of a luminous hazeover everything. We shall endeavour to exemplify each ofthese. (a) The first is well sho^n in the image of Peristephaniaeutycha when examined through th
The London, Edinburgh and Dublin philosophical magazine and journal of science . l be found instructive. 40. Intercostal Markings and allied phenomena. See § 15,p. 345.—Light which does not contribute to delineate any-thing upon the object is apt to intrude in three forms—either (a) concentrated into patterns which are superposedupon the microscopic image, or (b) scattered in patches overparts of it, or (r) spread in the form of a luminous hazeover everything. We shall endeavour to exemplify each ofthese. (a) The first is well sho^n in the image of Peristephaniaeutycha when examined through the half-inch apochromaticobjective. 1 he real detail upon this diatom seems to consistof divisions into hexagonal cells which make it look like ahoneycomb. Adjust the draw-tube accurately. Now nearlyclose the lower iris diaphragm, and look at image x. Thedioptric light makes a central white macula, and round it aredisplayed, as in the figure, a beautiful an ay of colouredmacula?, each of which is a spectrum, blue inwards and red Dr. G. J. Stoney on Microscopic Vision. 521. outwards, To make the following experiment, the loweriris diaphragm should be opened to so moderate an extentas will cause these maculae just tocome short of touching one another. When now the Daviss shutter isgradually opened, which increases theaperture of the objective, a successionof images present themselves. If itcovers up all the maculae except thecentral one, the diatom is indeed seen,but with no detail upon it. In allsubsequent enlargements of the aper-ture, the honeycomb structure of thediatom is visible ; but with the ad-dition of spurious effects which vary * while the grasp of theobjective is being enlarged. In fact, they depend on whatspectra are permitted to pass and what spectra are excluded. Let us consider three of these images, I, 2, and 3, ofwhich 2 is formed with a larger aperture than 1, and 3 witha larger aperture than 2. If we start with image 1 and openthe Daviss shutter so as to
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