1892.J THE MICEOSCOFE. 241 



123. / have a7i Abbe coiidenser zvith a B. and L. siibstage 

 aiiac/unent. Hoiv shall 1 7?ianage it to produce a dark-Jield 

 illufnination ? Where should the light be placed P Should 

 the diaphragfus be perfectly circular ? — A. E. S. 



The condenser must be accurately centred with the micro- 

 scope — a cap with a minute central hole fitting over the top lens 

 is usually provided ; use an inch objective on the microscope ; 

 place the cap on the condenser, focus the objective on this cap 

 and bring the small hole to the centre of the field by means of the 

 substage centering screws or other adjustment on the substage 

 ring. The lamp should stand conveniently near, say about iS 

 inches, preferably to the left. Use the plane mirror to reflect the 

 light into the condenser ; remove the perforated cap when the 

 condenser has been centred. For dark-field illumination, remove 

 all diaphragms or open wide the iris diaphragm (if one is at- 

 tached), and slip into place a central stop — that is. a diaphragm 

 with the centre stopped, and an annular marginal opening ; move 

 the condenser up or down until the object is properly illuminated. 



Each objective requires a suitable position of the condenser, and 

 wide angle objectives will perform better with wide diaphragm 

 openings ; a large condensing lens placed at its focal distance 

 from the lamp flame so as to throw parallel rays upon the mirror 

 will largely prevent color fringes in the images ; a white sun-lit 

 cloud is the best illuminant, but not always available. The dia- 

 phragms should be circular, but a slight irregularity will not be 

 important for ordinary work ; practise with the condenser, using 

 a familiar mount and diflerent objectives. Diflusion of the light 

 by lowering the condenser does not give as sharp definition as a 

 close position under the mount, using a small central opening 

 suited to the objective ; with verv wnde angled objectives make 

 an immersion contact between the top lens of the condenser and 

 the mount with a drop of glycerine or of homogeneous immersion 

 fluid for best results. For resolutions of difiicult diatoms a 

 diaphragm with a hole near one margin should be used to throw 

 a very oblique pencil of light from a direction at right angles to 

 the stride or markings on the object. 



124. What microscopical zvork ca7i be done i^i winter by an 

 amateur ivho has no 7naterial ? — H. U. S. 



Go to the nearest pond or permanently wetditchand gather half 

 a pint of bottom mud and two quarts of the \vater ; place all in a 

 glass jar in a north window. In a few davs life will develop 

 from spores and eggs. Diatoms, rotifers, cyclops, worms, moss, 

 etc., will afford objects for study. Mount and study the different 

 textile fibres — cotton, silk, wool, mohair, jute, hemp, flax, etc. 

 The small branches of woody plants may be sectioned with a 

 razor and studied. Prepare mounts of chemical crystals and 



