APPENDICES 



153 



Under-exposure with prolonged development 

 to gain density will often produce a dull 

 picture, but then it is usually " chalky." 



Unsuitable plates may cause this effect too ; 

 one developed with ammonia, if in excess ; and 

 this happens not unfrequently with iso- 

 chromatic plates. Avoid ammonia in developer. 



3. Definition good on one side of the picture, 3. This may arise from the specimen not 



but poor on the other. being flat, or the cover-glass uneven or bent. 



The objective may be out of centre (see 

 page 74). The draw-tube may be out of cen- 

 trality. The slip upon which the specimen 

 is placed unequal in thickness. If with low- 

 power work, the support S in Fig. 9, page 14, 

 not at right angles with the lens ; or the lens 

 and camera not at right angles with the stage 

 S. The lens may not be mounted true on the 

 camera. The camera hack which holds the 

 dark slide may be out of square with the 

 camera. The dark slide may itself be at fault. 

 Try a piece of ground glass in the slide itself, 

 and examine the picture attentively; the 

 plate itself may be at fault by being thicker at 

 one end than the other, but this is a mere 

 possibility in the present day. 



4. Bad focussing through ground glass not 

 in register with dark slide, or fault in the 

 actual focussing to commence with. If a hand 

 focusser be used, it may not have been pre- 

 viously adjusted to the thickness of the ground 

 glass (see page 55). A bad objective in which 

 the chemical and visual foci may not be co- 

 incident : a visual achromatic used in the place 

 of one corrected for photography : a lens em- 

 ployed which will not photograph correctly 

 with a given colour-screen : a slight shake in 

 the apparatus at the time of photographing — 

 with very high-power work, a piece of the 

 lime flying off during the exposure. The fine 

 adjustment out of order jumping after operator 

 has finished. 



4. General definition at fault in the nega- 

 tive. 



