114 APPENDAGES TO THE MICROSCOPE. 



The Diaphragms for use with this or with the Webster Condenser, 

 when very oblique illumination is required, may be cut out of 

 thin brass or tin-foil, and blackened with oxide of copper. The 

 apertures should be V~ sna P e d\ extending from the circumference 

 to about a quarter of an inch from the centre ; and it is often 

 useful to have two such apertures in the same diaphragm at angles 

 of from 60° to 90° from each other, so that two pencils of light may 

 fall at the same time in different directions upon two sets of lines. 

 By an ingenious arrangement devised by Mr. Reade, a second 

 adjustable diaphragm may be made to shut off the inner portions 

 of the V" s h a P e d apertures, leaving only such parts of their 

 marginal portions as may give the required obliquity to the 

 illuminating rays.* 



84. Black-Ground Illuminators. — Whenever the rays are 

 directed with such obliquity as not to be received into the Object- 

 glass at all, but are sufficiently retained by the Object to render it 

 (so to speak) self-luminous, we have what is known as the black- 

 ground illumination. For low powers whose angular aperture is 

 small, and for such objects as do not require any more special 

 provision, a sufficiently good ' black-ground ' illumination may be 

 obtained by turning the concave Mirror as far as possible out of 

 the axis of the microscope, especially if it be so mounted as to be 

 capable of a more than ordinary degree of obliquity. In this 

 manner it is often possible, not merely to bring into view features 

 of structure that might not otherwise be distinguishable, but to 

 see bodies of extreme transparence (such, for instance, as very 

 minute Animalcules) that are not visible when the field is flooded 

 (so to speak) by direct light ; these presenting the beautiful spec- 

 tacle of phosphorescent points rapidly sailing through a dark 

 ocean. Another very simple mode, which answers sufficiently well 

 for low powers and for the larger objecfs which these are fitted to 

 view, consists in substituting for the ordinary Condenser a plano- 

 convex lens of great convexity, having on its plane side, which is 

 the one turned towards the object, a central stop to cut off the 

 direct rays ; for the rays passing through the marginal portion of 

 this Spot-Lens, being strongly refracted by its high curvature, 

 are made to converge upon the object at an angle too wide for 

 their entrance into an Objective of moderate aperture, and thus the 

 field is left dark ; whilst all the light stopped by the object serves 

 (as it were) to give it a luminosity of its own. The same effect is 

 gained by the use of the Webster Condenser (§ SO) with a central 

 stop placed immediately behind the lower lens or upon the flat 

 surface of the upper. Neither of the foregoing plans, however, 



* See "Transactions of Microscopical Society," Vol. xv. p. 3. — Another 

 Illuminator, giving a wide angular pencil, and specially devised by- 

 Mr. "VVenham for use with the Binocular Microscope, is described by 

 him in "Quart. Journ. of Microsc. Science," Vol. i. N.S. (1861), p. 111. 



