1900] MICROSCOPICAL JOURNAL. 135 



would caution beginners against the too prevalent tend- 

 ency to cut off an unnecessary portion of the marginal 

 rays in order to secure contrast in the image. By so do- 

 ing, the latter suffers rather than gains, diffraction im- 

 ages being set up. As a matter of fact, the diaphragm 

 should be closed gently and cautiously, just enough to 

 slightly accentuate the image and no more ; and it will gen- 

 erally be found that, under these circumstances, if the 

 eye-piece be removed, the back of the objective, examin- 

 ed down the empty tube, is found to be from § to f filled 

 with light. A good objective should stand satisfactorily 

 what is accordingly often spoken of as a I cone of illumin- 

 ation. 



In most cases strictly axial, i. e. central, illumination is 

 used ; but there are certain cases, such for instance, as the 

 structure of diatoms, for which oblique illumination is 

 necessary. Formerly this was obtained by swinging the 

 mirror, or mirror and condenser combined, slightly to one 

 side, but more generally now by the insertion of a good 

 stop in the carrier beneath the condenser. In this stop a 

 narrow slot has been cut, or a segment cut out. The effect 

 of this is to throw an oblique beam of light in one direc- 

 tion across or down the diatom, according to arrange- 

 ment, and thus to bring into greater prominence shadows 

 of the fine striae otherwise almost invisible, owing to their 

 exceeding minuteness. To do this properly requires con- 

 siderable practice and experience, as may be readily un- 

 derstood when we remember that the "markings" in Am- 

 phipleura pellucida exceed 90,000 to the inch. 



We have alluded to the necessity of making sure be- 

 forehand that the mirror and tail-rod are truly in line with 

 the optic axis of the microscope. If this be not the case, 

 the image of the lamp flame will appear to shift in posi- 

 tion as the tube of the microscope is racked up and down. 

 It is necessary, in addition, that the condenser itself shall 

 be truly centered with the objective. Microscopes fitted 



