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light passing through the aperture and lens, it will be seen that the 

 outline of the shadow is surrounded by a coloured fringe. This is 

 the visible effect of the interference of light. The same may be 

 shown without the lens, but not so plainly. 



The intended limits of this paper will not allow me to go into the 

 details of these phenomena, therefore I shall only mention the effects 

 of interference observed under the microscope. It will be seen that 

 there is a faint band or fringe surrounding minute objects seen 

 under high powers, by ordinary illumination, which prevents the 

 outline from being distinctly marked : even under our very best 

 glasses the effect of this interference is seen around the margin of a 

 minute filament or hair, but most especially by the Monas termo, the 

 smallest of the Infusoria, which appears as a little, moving, nebulous 

 spot of light, which when shown by its own radiant light, produced 

 on this new principle, is perfectly developed, both for outline and 

 colour. The effect of this interference on some transparent Infu- 

 soria and other objects, is to blend together the complex internal 

 organization, which they ought to display when seen only with their 

 own light, that, from the source of illumination, being prevented 

 from entering the object-glass. 



As the method of illuminating microscopic objects by means of a 

 very large angular pencil of light, having the central rays obscured, 

 is but just introduced and therefore a novelty, I shall mention in 

 corroboration a few instances where transparent objects are shown, 

 under similar circumstances, with perfect or improved definition. 



If a small black wafer be fixed to a window, against the brightest 

 part of the sky, and a large transparent object, such as a feather or 

 the wing of a dragon-fly, be held before it, and examined with a 

 hand-magnifier, of about an inch focus, it will then be entirely 

 illuminated by lateral light, and it will be seen that the object is 

 more beautifully developed than when shown against a bright cloud, 

 the comparison being immediate. 



Also it may be observed that the structure of a muslin curtain 

 immediately before the bar of a window is more readily ascertained 

 than when directly opposed to bright skylight. 



Those who are in the habit of collecting Infusoria will be assisted 

 in distinguishing the various species, if a slip of black cloth be 

 cemented to the further side of the phial, which should be held 

 directly against the sky, and the contents examined with a Cod- 

 dington lens or hand-magnifier. 



Other examples, under our every-day observation, might be cited 



