THE MICROSCOPE. : 97 
to find an object under a low power, and, placing it in the cen- 
tre of the field, it is in position when the higher powers are 
employed. 
In focussing the microscope it is better first to run the tube 
down toward the slide to within the focal limits of the objec- 
tive, watching the operation from the side and seeing that 
the cover glass is not touched. Then, with the eye to the 
eye-piece of the microscope, the tube is slowly moved back 
by the coarse adjustment until a good view of the object is 
obtained, and then the fine adjustment is used. 
In using immersion objectives a drop of water is put on 
the front of the objective which is then placed on the mi- 
croscope and run down to the cover glass so that the water 
forms a thin film between the objective and the cover glass. 
A very simple experiment, for which I am indebted to Mr. 
Phin,? shows how the immersion aids in the defining power 
of the microscope. ‘Take four ordinary plate glass slides 
and place a very small drop of watcr in the centre of three 
of them. Across the ends of these three slides lay a narrow 
strip of stout writing paper, and then place the four slides 
together so that between every two there shall be a drop of 
water and also two slips of paper to keep them apart.” If 
you now look through these four slides at any object, the 
spots where the three drops of water have been placed will 
look like a hole it will appear so clean and _ transparent.” 
The water produces exactly the same effect with the immer- 
3 Practical Hints on the Selection and Use of the Microscope, p. 38, by John 
Phin, N. Y., 1875. 
7 
