98 ZOOLOGICAL EXERCISES. 
sometimes get dirty by focussing them down into the fluid 
containing the object. This must be removed by breathing 
on the objective, and rubbing with a handkerchief. When 
the object cannot be brought out distinctly, the state of the 
objective should always be examined. 
The Work Taste. This should be strong and steady, 
and not less than three feet by two. If work is done by 
daylight it should be placed in front of a window that is 
not exposed during working hours to the direct rays of the 
sun. <A dissecting microscope formed by a single lens 
Supported on an arm is necessary. If the work is at night, 
a lamp and a bull’s-eye condenser must also be provided. 
The dissecting microscope should be placed on the middle of 
the table; the compound microscope on the right. In 
working at night the lamp should be placed in front of the 
dissecting microscope and ten or twelve inches from it. The 
condenser should be close (between two and three inches) to 
the lamp, with its flat side turned to it, and arranged to 
throw a parallel beam of light on to the dissecting trough. 
The mirror of the compound microscope must be also 
suitably arranged. A microscope lamp with a dead white 
porcelain shade is very convenient, not only can it be set at 
any height required, but the shade protects the eyes from 
the light, and the head from the heat. 
EXERCISE I. 
ad, Take a glass slide and clean it carefully with a linen 
handkerchief. Then take a thin class cover and clean 
it with the handkerchief between the finger and 
thumb of the right hand. It requires a little caution 
not to break the glass. With the handle of a 
