1886-87.] as showing Peculiarities of certain Diatoms. 57 



may be condeused into two necessary conditions : first, that 

 the object is iridescent; and second, that it is mounted in a 

 reasonably thin medium. 



There are several kinds of micro-accessories in general use for 

 the production of dark-ground illumination, but I shall confine 

 myself to a description of those that I have tried : — 



I. Bi/ Means of an ordinary Stand- Condenser. — The lamp is 

 placed so that the flame is about the same height as the stage, 

 and the edge of the wick turned towards the microscope. All 

 sub-stage arrangements are removed, so as to allow the light 

 greater range, and the mirror turned aside or taken off altogether. 

 The condenser is now placed, and the microscope tilted to allow 

 the rays to strike the object upon the slide, and then the con-, 

 denser is moved until the rays are brought to a focus upon the 

 object. After the rays pass the conjugate focus at the stage, 

 they diverge, and do not enter the objective at all. For, pro- 

 vided they fall upon the under side of the glass slip at a reason- 

 able angle, they will pass onwards in their original course ; but 

 should they impinge at an angle exceeding, say, 70°, they will, if 

 properly focussed, be almost totally reflected downwards, — so 

 that in the first case they strike the object as they pass, al- 

 though not entering the objective ; while in the other case they 

 do not properly illuminate the object. It is, therefore, mani- 

 festly important that the slope of the microscope should not be 

 such as to cause a loss of light by reflection. Granted, therefore, 

 that the object under examination is an iridescent one, the ob- 

 server will see it by reflected light, which passes up from it 

 into the objective. The object will thus stand upon a dark 

 ground, illuminated and examined according to the same optical 

 principles as the moon or the stars. It is important that the 

 focal length of the objective be taken into account, in order to 

 procure the best results from this kind of illumination. Sup- 

 pose the objective to be a high power with short working dis- 

 tance, it will be found practically impossible to exclude the 

 direct rays from entering the objective ; but I have found that 

 even with a high -angled J-inch it is quite possible to get dark- 

 ground illumination from the stand-condenser. 



II. By Means of a Spot - Lens. — This instrument may be 

 generally described as a ijlano-convex lens with a slice taken 

 off the convexity, and the flat face thus formed rendered 



