PLATE 12. 



Some faults commonly met with. 



Diffraction. 



A and B HAIRS ON THE WING OF HONEY BEE x 186 



Objective Leitz No. 7. N. A. 0.85. 



Ocular None. 



Condenser ... Watson's Universal. 

 Cond. Aperture A. N. A. 0.75. 



B. Iris closed down very greatly. 



A The hairs are sharply defined, but owing to the small penetra- 

 tion of the objective at the large aperture, the bases of the hairs, and 

 hairs in other planes are out of focus. 



B Restriction of the aperture of the condenser causes white line 

 diffraction effects and duplication of the points of the hairs. The 

 penetration of the objective is greatly increased at the low N.A., and 

 hairs previously out of focus become visible. 



D DIATOM Navicula crabro x 400 



Objective Swift I". N. A. 0.88. 



Ocular Swift No. 2. 



Condenser ... Watson's Universal. 



Cond. Aperture The print is composite from tw r o negatives, the 

 left half being taken at nearly open aper- 

 ture (N.A. 0.75), the right with iris greatly 

 closed. 



Diffraction effects are visible as a series of concentric lines round 

 the diatom, arid specks of dust invisible with the open aperture 

 become apparent as small circles when the N.A. is restricted. 



Flare Spot. 



C GROUP OF DIATOMS x 60 



The white flare spot in 'the centre is due to reflections from bright 

 portions of the interior of the microscope tube, no eye-piece being 

 used . 



Faulty definition, and lack or excess of contrast, due to the use of 

 unsuitable plates or colour screens, are illustrated on Plates 17 to 21. 



[To follow page 120. 



