1900] MICROSCOPICAL JOURNAL. 137 



fers somewhat, therefore, for lamplight and for daylight. 



We have already alluded to the use of oblique illumin- 

 ation by means of suitable spots placed beneath the con- 

 denser in a carrier adapted for the purpose. There is an- 

 other form of illumination generally spoken of as "dark- 

 ground illumination." In this case a small ceatral spot, 

 supported on arms, occupies a similar position, and by cut- 

 ting off those central rays of light which would otherwise 

 enter the objective, gives an absolutely dark field in the 

 microscope. When a suitable object is interposed on the 

 stage, however, the annular rays of light that would other- 

 wise escape the objective are intercepted by the object, 

 and thus diffracted into view. The result is often singu- 

 larly beautiful — diatoms, foraminifera, and transparent 

 zoophytes being exhibited shining upon an otherwise dark 

 and contrasting background. To obtain the best results 

 it is necessary to bear in mind that the diameter of the 

 stop must be proportioned to the aperture of the object- 

 ive. Thus a low-powered, and presumably low-apertured, 

 lens will require a small stop, whilst a high-powered and 

 high-apertured lens will require a much larger stop. It 

 would not be of much advantage if we were to give means 

 of ascertaining the size of stop required ; if necessary, an 

 experimental stop can readily be constructed out of black- 

 ened cardboard. The condenser will also require a cer- 

 tain amount of adjustment ; and a Davis shutter, which is 

 a small iris diaphragm fitted above the objective, by itg 

 facility for reducing the aperture of the objective, enables 

 one to obtain perfect background illumination. The spot 

 lens and parabolic illuminator are now but little used. 

 The value of this means of illumination for really critical 

 work we need not discuss here. The mere slight tilting of 

 the mirror will often greatly increase the resolution of 

 difficult objects. 



Larva of Spongilla Lacustris. — The "Quarterly Jour- 

 nal of Microscopical Science" is occupied to the extent of 



