CHAPTER VI. 
ILLUMINATION. 
O UNLIGHT is undoubtedly the best radiant for the 
^ photo-micrographer, when he can get it. But in 
this country direct sunlight is so rarely to be depended 
upon, that if we were to provide apparatus for the 
utilisation of this light alone we might yearly pass 
many days without making a single exposure. Diffused 
daylight, however fine in quality for general photo¬ 
graphy, is of little service to the photo-micrographer, 
who requires rays more or less direct. But direct 
sunlight contains so large an amount of the rays most 
useful to us, and the rays most suitable for photo¬ 
graphic purposes are so easily separated out from the 
other rays, a large amount of photographic light being 
still available for even very short exposures, that sun¬ 
light should not be wholly overlooked in a manual such 
as this. 
If we propose to use sunlight we shall almost cer¬ 
tainly require a heliostat of some kind. This instru¬ 
ment enables us to keep a reflected image of the sun 
shining steadily in one direction in spite of the apparent 
motion of the sun. The heliostat (fig. 23) is as good as 
any we have seen or used. 
