APPENDICES 



156 



21. In low-power work ; adopting all pre- 

 cautions explained, if the light falls off sensibly 

 along the periphery of picture, although defini- 

 tion not so much impaired. 



22. Absence of UacJcness in the shadows. 



23. The clear portions of specimen are 

 clouded and veiled in the print. 



24. Small dots on diatoms or faint lines 

 imperfectly shown. 



25. Eoundness of field. 



21. Lens has not sufficient " covering power." 

 Use one of longer focus, or having a wider 

 angle. 



22. Nearly always from a badly corrected 

 objective ; occasionally from " flooding " with 

 too much light. Try contracting the cone a 

 trifle by closing the iris. 



23. Over-exposure in print, or objective 

 badly corrected for colour. Try intensifying 

 the negative. May be due to over-exposure. 



24. Too low a N. A. in objective, or too much 

 flooding of light. Badly corrected objective : 

 bad centering of the apparatus somewhere. 



25. Apochromats of the very highest order 

 have this fault. Use a lower eye-piece and draw 

 out camera to obtain necessary amplification. 

 Always remember to use a low eye-piece (pro- 

 jection 6 is best), and lengthen camera when- 

 ever possible. 



