30 PHOTO-MICROGRAPHS. 
ent times and in different places. It is less in 
northern latitudes and during the shortest days, 
when the sun’s rays reach the earth in a very 
oblique direction. It fails rapidly as the sun sinks 
in the heavens towards evening. It is greatest 
when the atmosphere is transparent, and distant 
objects are distinctly seen. On the other hand, a 
hazy condition of the atmosphere indicates that 
a multitude of minute particles are suspended in 
it, and these being interposed between our mirror 
—the sky—and the object, obstruct and weaken 
the light. 
The smoke of factories, etc., even when greatly 
attenuated, is a veil in front of our mirror which 
is fatal to any attempt to work except with the 
lowest powers, and even with these it introduces 
an element of uncertainty which leads to much 
trouble. 
This smoke and haze is commonly confined to 
the lower regions of the atmosphere, and if our 
instrument is placed horizontally, so as to receive 
light reflected from the sky at a point near the 
horizon, we have the disadvantage of having a 
thick stratum of this hazy atmosphere interposed 
in front of our mirror. Moreover, unless we are 
in an elevated situation, a horizontal position of 
the instrument is apt to cause the condensing lens 
to face some terrestrial object, instead of the clear 
blue sky. It is therefore generally necessary to 
place the instrument obliquely with reference to 
the surface of the earth, in the position of a tele- 
scope, and the elevation must be greater or less, 
according to circumstances. 
