TESTING - 343 
keep the wick of a wet-and-dry-bulb hygrometer in good condition 
in the incubator, because the high temperature will quickly dry 
it out and make frequent changes of the wick necessary. For 
practical purposes, the spiral or horse-hair hygrometer is much 
more satisfactory. To the inexperienced operator, however, the 
increasing size of the air cell will be the safest guide. \ 
Testing.—To determine the fertility of the eggs, as well as to 
study the developing embryos and thus ascertain whether the 
machine is running properly, it is advisable to test or candle the eggs 
once or, better, twice during 
the hatch, preferably on the 
seventh and fourteenth days. 
The egg tray should be re- 
moved to a dark testing room, 
and the light for testing pro- 
vided either by a kerosene lamp 
or an acetylene or electric light 
placed in a small tight box 
with circular opening about 
one inch in diameter. Electric 
light is the best, the light from 
a kerosene lamp not being 
powerful enough to penetrate 
the shell of the egg (Fig. 161). 
The egg tray should be 
placed on the right-hand side 
of the lamp box, the person 
standing in front, with a du- Tic: JO. 4 welth home of ete 
plicate empty tray at his left testing of two eggs at once. 
hand in which to put the eggs 
as tested. The testing lamp should be so placed that the open- 
ing is about six inches above the waist line and one foot in 
front of the operator. The untested eggs should be taken two 
or three at a time from the full tray, and transferred one at a 
time to the other hand, grasping them between the thumb and 
forefinger with the large or air cell end outward. As the eggs are 
moved, they are brought one at a time in front of the opening, 
and given a gentle rotary motion. This will move the contents, and 
the light penetrating the shell will reveal the presence or absence 
of the germ, and its condition. The chief points to be determined 
in the seventh day’s test are the size and location of the air cell, 
