32 ELEMENTARY PHOTO-MICROGRAPHY. 



fore find some means of diffusing it over the whole 

 surface of the object, and this can be done by inter- 

 posing the bull's eye. The lamp is again reversed 

 with the bull's eye previously set in focus, and an 

 evenly-lit disc on the cardboard is sought as in the 

 method last described. Having got it satisfactorily, 

 the central rays are stopped out by means of a disc 

 in the diaphragm plate, so that an annular ring of 

 light only enters the condenser. 



If it be diatoms we are photographing they will 

 now be seen glittering like pearls upon black 

 velvet. The microscope is connected up, focussed, 

 and an exposure made. Longer exposure must be 

 given than with direct light. The mirror of the 

 microscope is not required. It is well to make sure 

 that the substage condenser is properly centred with 

 the optical axis of the microscope before com- 

 mencing. 



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 dia- 



