72 PHOTO-MICROGRAPHS. 
position of the achromatic condenser, when one is 
employed ; the power and angle of aperture of the 
objective; the use, or otherwise, of an amplifier ; 
the distance of the screen. 
When direct sunlight, reflected by a heliostat, 
is employed, the exposure with the extremely 
sensitive dry plates is very short, and with low 
powers it will be necessary to resort to the use of 
a mechanical contrivance by which the plate is 
exposed for the fraction of a second. This consists 
of a “drop-screen” placed between the blue-cell 
and the achromatic condenser, which has an ad- 
justable rectangular opening in the middle line, 
which admits the light to the plate during the 
time occupied in passing in front of the line of 
light. 
In general terms it may be said that, The actinic 
power of the light ts less when the sun ts low in the 
heavens, and when the atmosphere ts hazy. 
Light is always lost by the use of reflecting surfaces 
or refracting media ; consequently the more lenses 
it has to pass through on its way to the screen, the 
weaker it will be, independently of the attenua- 
tion due to divergence of the rays. 
Low powers admit more light, because of the 
greater diameter of the front lens, which affords 
a greater area for admission of light. In lenses 
of the same magnifying power, the comparative 
amount of light admitted depends upon the angle 
of aperture. An immersion-lens admits more light 
than a dry-lens of the same angle; a homogeneous 
immersion objective, more light than one used with 
