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 v/hich I am indebted to Mr. 

 Phin, 3 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 water 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- 



Practical Hints on the Selection and Use of the Microscope, p. 38, by John 

 Pbin,N.Y.,i8 7 5. 

 7 



