ELEMENTARY PHOTO-MICROGRAPHY. 53 



best possible definition and illumination will be got. 

 But when the condenser is out of centre and out 

 of focus, the rays of light will be dispersed use- 

 lessly, as seen in fig. 20. 



Some negatives that yield prints on a light 

 ground may also be used for printing with a black 

 ground by simply making a positive from such a 

 one and using this to print from. 



Fig. 21 (Plate VI.) is a light ground print from 

 a negative taken without any central stop in the 

 condenser, and fig. 22 is a print from the glass 

 positive which gives the dark ground effect. 



When photographing without the eyepiece, a 

 bright spot in the centre of the illuminated disc 

 will appear if the body tube be not lined with black 

 velvet or otherwise dead blacked. This is caused 

 by reflection, and is specially noticeable when using 

 polarised light, but the eyepiece removes this 

 defect entirely, though with greatly prolonged 

 exposure. 



Occasionally the substage condenser may be 

 lowered with advantage, and a more even illumina- 

 tion secured, although at a sacrifice of brightness. 

 When the markings of certain diatoms are to be 

 seen at their best, the light should be thrown on 

 them obliquely by means of a crescent-shaped 

 diaphragm, taking care that the lighting of the 

 ground-glass screen is both uniform and sufficient 

 to ensure a background of tolerable density. 



